diff --git a/changelog.d/7546.misc b/changelog.d/7546.misc
new file mode 100644
index 000000000..0a9704789
--- /dev/null
+++ b/changelog.d/7546.misc
@@ -0,0 +1 @@
+Optimise some references to `hs.config`.
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index ea25f0515..681f92caf 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -366,7 +366,9 @@ class EventCreationHandler(object):
         self.notifier = hs.get_notifier()
         self.config = hs.config
         self.require_membership_for_aliases = hs.config.require_membership_for_aliases
-        self._instance_name = hs.get_instance_name()
+        self._is_event_writer = (
+            self.config.worker.writers.events == hs.get_instance_name()
+        )
 
         self.room_invite_state_types = self.hs.config.room_invite_state_types
 
@@ -836,7 +838,7 @@ class EventCreationHandler(object):
         success = False
         try:
             # If we're a worker we need to hit out to the master.
-            if self.config.worker.writers.events != self._instance_name:
+            if not self._is_event_writer:
                 result = await self.send_event(
                     instance_name=self.config.worker.writers.events,
                     event_id=event.event_id,
@@ -906,7 +908,7 @@ class EventCreationHandler(object):
 
         This should only be run on the instance in charge of persisting events.
         """
-        assert self.config.worker.writers.events == self._instance_name
+        assert self._is_event_writer
 
         if ratelimit:
             # We check if this is a room admin redacting an event so that we
diff --git a/synapse/server_notices/resource_limits_server_notices.py b/synapse/server_notices/resource_limits_server_notices.py
index d97166351..73f2cedb5 100644
--- a/synapse/server_notices/resource_limits_server_notices.py
+++ b/synapse/server_notices/resource_limits_server_notices.py
@@ -48,6 +48,12 @@ class ResourceLimitsServerNotices(object):
 
         self._notifier = hs.get_notifier()
 
+        self._enabled = (
+            hs.config.limit_usage_by_mau
+            and self._server_notices_manager.is_enabled()
+            and not hs.config.hs_disabled
+        )
+
     async def maybe_send_server_notice_to_user(self, user_id):
         """Check if we need to send a notice to this user, this will be true in
         two cases.
@@ -61,14 +67,7 @@ class ResourceLimitsServerNotices(object):
         Returns:
             Deferred
         """
-        if self._config.hs_disabled is True:
-            return
-
-        if self._config.limit_usage_by_mau is False:
-            return
-
-        if not self._server_notices_manager.is_enabled():
-            # Don't try and send server notices unless they've been enabled
+        if not self._enabled:
             return
 
         timestamp = await self._store.user_last_seen_monthly_active(user_id)
diff --git a/synapse/storage/data_stores/main/monthly_active_users.py b/synapse/storage/data_stores/main/monthly_active_users.py
index 925bc5691..a42c52f32 100644
--- a/synapse/storage/data_stores/main/monthly_active_users.py
+++ b/synapse/storage/data_stores/main/monthly_active_users.py
@@ -122,6 +122,10 @@ class MonthlyActiveUsersStore(MonthlyActiveUsersWorkerStore):
     def __init__(self, database: Database, db_conn, hs):
         super(MonthlyActiveUsersStore, self).__init__(database, db_conn, hs)
 
+        self._limit_usage_by_mau = hs.config.limit_usage_by_mau
+        self._mau_stats_only = hs.config.mau_stats_only
+        self._max_mau_value = hs.config.max_mau_value
+
         # Do not add more reserved users than the total allowable number
         # cur = LoggingTransaction(
         self.db.new_transaction(
@@ -130,7 +134,7 @@ class MonthlyActiveUsersStore(MonthlyActiveUsersWorkerStore):
             [],
             [],
             self._initialise_reserved_users,
-            hs.config.mau_limits_reserved_threepids[: self.hs.config.max_mau_value],
+            hs.config.mau_limits_reserved_threepids[: self._max_mau_value],
         )
 
     def _initialise_reserved_users(self, txn, threepids):
@@ -142,6 +146,15 @@ class MonthlyActiveUsersStore(MonthlyActiveUsersWorkerStore):
             threepids (list[dict]): List of threepid dicts to reserve
         """
 
+        # XXX what is this function trying to achieve?  It upserts into
+        # monthly_active_users for each *registered* reserved mau user, but why?
+        #
+        #  - shouldn't there already be an entry for each reserved user (at least
+        #    if they have been active recently)?
+        #
+        #  - if it's important that the timestamp is kept up to date, why do we only
+        #    run this at startup?
+
         for tp in threepids:
             user_id = self.get_user_id_by_threepid_txn(txn, tp["medium"], tp["address"])
 
@@ -191,8 +204,7 @@ class MonthlyActiveUsersStore(MonthlyActiveUsersWorkerStore):
 
             txn.execute(sql, query_args)
 
-            max_mau_value = self.hs.config.max_mau_value
-            if self.hs.config.limit_usage_by_mau:
+            if self._limit_usage_by_mau:
                 # If MAU user count still exceeds the MAU threshold, then delete on
                 # a least recently active basis.
                 # Note it is not possible to write this query using OFFSET due to
@@ -210,13 +222,13 @@ class MonthlyActiveUsersStore(MonthlyActiveUsersWorkerStore):
                             LIMIT ?
                         )
                         """
-                    txn.execute(sql, (max_mau_value,))
+                    txn.execute(sql, ((self._max_mau_value),))
                 # Need if/else since 'AND user_id NOT IN ({})' fails on Postgres
                 # when len(reserved_users) == 0. Works fine on sqlite.
                 else:
                     # Must be >= 0 for postgres
                     num_of_non_reserved_users_to_remove = max(
-                        max_mau_value - len(reserved_users), 0
+                        self._max_mau_value - len(reserved_users), 0
                     )
 
                     # It is important to filter reserved users twice to guard
@@ -335,7 +347,7 @@ class MonthlyActiveUsersStore(MonthlyActiveUsersWorkerStore):
         Args:
             user_id(str): the user_id to query
         """
-        if self.hs.config.limit_usage_by_mau or self.hs.config.mau_stats_only:
+        if self._limit_usage_by_mau or self._mau_stats_only:
             # Trial users and guests should not be included as part of MAU group
             is_guest = yield self.is_guest(user_id)
             if is_guest:
@@ -356,11 +368,11 @@ class MonthlyActiveUsersStore(MonthlyActiveUsersWorkerStore):
                 # In the case where mau_stats_only is True and limit_usage_by_mau is
                 # False, there is no point in checking get_monthly_active_count - it
                 # adds no value and will break the logic if max_mau_value is exceeded.
-                if not self.hs.config.limit_usage_by_mau:
+                if not self._limit_usage_by_mau:
                     yield self.upsert_monthly_active_user(user_id)
                 else:
                     count = yield self.get_monthly_active_count()
-                    if count < self.hs.config.max_mau_value:
+                    if count < self._max_mau_value:
                         yield self.upsert_monthly_active_user(user_id)
             elif now - last_seen_timestamp > LAST_SEEN_GRANULARITY:
                 yield self.upsert_monthly_active_user(user_id)
diff --git a/tests/server_notices/test_resource_limits_server_notices.py b/tests/server_notices/test_resource_limits_server_notices.py
index 406f29a7c..99908edba 100644
--- a/tests/server_notices/test_resource_limits_server_notices.py
+++ b/tests/server_notices/test_resource_limits_server_notices.py
@@ -27,20 +27,33 @@ from synapse.server_notices.resource_limits_server_notices import (
 )
 
 from tests import unittest
+from tests.unittest import override_config
+from tests.utils import default_config
 
 
 class TestResourceLimitsServerNotices(unittest.HomeserverTestCase):
-    def make_homeserver(self, reactor, clock):
-        hs_config = self.default_config()
-        hs_config["server_notices"] = {
-            "system_mxid_localpart": "server",
-            "system_mxid_display_name": "test display name",
-            "system_mxid_avatar_url": None,
-            "room_name": "Server Notices",
-        }
+    def default_config(self):
+        config = default_config("test")
 
-        hs = self.setup_test_homeserver(config=hs_config)
-        return hs
+        config.update(
+            {
+                "admin_contact": "mailto:user@test.com",
+                "limit_usage_by_mau": True,
+                "server_notices": {
+                    "system_mxid_localpart": "server",
+                    "system_mxid_display_name": "test display name",
+                    "system_mxid_avatar_url": None,
+                    "room_name": "Server Notices",
+                },
+            }
+        )
+
+        # apply any additional config which was specified via the override_config
+        # decorator.
+        if self._extra_config is not None:
+            config.update(self._extra_config)
+
+        return config
 
     def prepare(self, reactor, clock, hs):
         self.server_notices_sender = self.hs.get_server_notices_sender()
@@ -60,7 +73,6 @@ class TestResourceLimitsServerNotices(unittest.HomeserverTestCase):
         )
         self._send_notice = self._rlsn._server_notices_manager.send_notice
 
-        self.hs.config.limit_usage_by_mau = True
         self.user_id = "@user_id:test"
 
         self._rlsn._server_notices_manager.get_or_create_notice_room_for_user = Mock(
@@ -68,21 +80,17 @@ class TestResourceLimitsServerNotices(unittest.HomeserverTestCase):
         )
         self._rlsn._store.add_tag_to_room = Mock(return_value=defer.succeed(None))
         self._rlsn._store.get_tags_for_room = Mock(return_value=defer.succeed({}))
-        self.hs.config.admin_contact = "mailto:user@test.com"
-
-    def test_maybe_send_server_notice_to_user_flag_off(self):
-        """Tests cases where the flags indicate nothing to do"""
-        # test hs disabled case
-        self.hs.config.hs_disabled = True
 
+    @override_config({"hs_disabled": True})
+    def test_maybe_send_server_notice_disabled_hs(self):
+        """If the HS is disabled, we should not send notices"""
         self.get_success(self._rlsn.maybe_send_server_notice_to_user(self.user_id))
-
         self._send_notice.assert_not_called()
-        # Test when mau limiting disabled
-        self.hs.config.hs_disabled = False
-        self.hs.config.limit_usage_by_mau = False
-        self.get_success(self._rlsn.maybe_send_server_notice_to_user(self.user_id))
 
+    @override_config({"limit_usage_by_mau": False})
+    def test_maybe_send_server_notice_to_user_flag_off(self):
+        """If mau limiting is disabled, we should not send notices"""
+        self.get_success(self._rlsn.maybe_send_server_notice_to_user(self.user_id))
         self._send_notice.assert_not_called()
 
     def test_maybe_send_server_notice_to_user_remove_blocked_notice(self):
@@ -153,13 +161,12 @@ class TestResourceLimitsServerNotices(unittest.HomeserverTestCase):
 
         self._send_notice.assert_not_called()
 
+    @override_config({"mau_limit_alerting": False})
     def test_maybe_send_server_notice_when_alerting_suppressed_room_unblocked(self):
         """
         Test that when server is over MAU limit and alerting is suppressed, then
         an alert message is not sent into the room
         """
-        self.hs.config.mau_limit_alerting = False
-
         self._rlsn._auth.check_auth_blocking = Mock(
             return_value=defer.succeed(None),
             side_effect=ResourceLimitError(
@@ -170,12 +177,11 @@ class TestResourceLimitsServerNotices(unittest.HomeserverTestCase):
 
         self.assertEqual(self._send_notice.call_count, 0)
 
+    @override_config({"mau_limit_alerting": False})
     def test_check_hs_disabled_unaffected_by_mau_alert_suppression(self):
         """
         Test that when a server is disabled, that MAU limit alerting is ignored.
         """
-        self.hs.config.mau_limit_alerting = False
-
         self._rlsn._auth.check_auth_blocking = Mock(
             return_value=defer.succeed(None),
             side_effect=ResourceLimitError(
@@ -187,12 +193,12 @@ class TestResourceLimitsServerNotices(unittest.HomeserverTestCase):
         # Would be better to check contents, but 2 calls == set blocking event
         self.assertEqual(self._send_notice.call_count, 2)
 
+    @override_config({"mau_limit_alerting": False})
     def test_maybe_send_server_notice_when_alerting_suppressed_room_blocked(self):
         """
         When the room is already in a blocked state, test that when alerting
         is suppressed that the room is returned to an unblocked state.
         """
-        self.hs.config.mau_limit_alerting = False
         self._rlsn._auth.check_auth_blocking = Mock(
             return_value=defer.succeed(None),
             side_effect=ResourceLimitError(
diff --git a/tests/storage/test_client_ips.py b/tests/storage/test_client_ips.py
index bf674dd18..3b483bc7f 100644
--- a/tests/storage/test_client_ips.py
+++ b/tests/storage/test_client_ips.py
@@ -23,6 +23,7 @@ from synapse.http.site import XForwardedForRequest
 from synapse.rest.client.v1 import login
 
 from tests import unittest
+from tests.unittest import override_config
 
 
 class ClientIpStoreTestCase(unittest.HomeserverTestCase):
@@ -137,9 +138,8 @@ class ClientIpStoreTestCase(unittest.HomeserverTestCase):
             ],
         )
 
+    @override_config({"limit_usage_by_mau": False, "max_mau_value": 50})
     def test_disabled_monthly_active_user(self):
-        self.hs.config.limit_usage_by_mau = False
-        self.hs.config.max_mau_value = 50
         user_id = "@user:server"
         self.get_success(
             self.store.insert_client_ip(
@@ -149,9 +149,8 @@ class ClientIpStoreTestCase(unittest.HomeserverTestCase):
         active = self.get_success(self.store.user_last_seen_monthly_active(user_id))
         self.assertFalse(active)
 
+    @override_config({"limit_usage_by_mau": True, "max_mau_value": 50})
     def test_adding_monthly_active_user_when_full(self):
-        self.hs.config.limit_usage_by_mau = True
-        self.hs.config.max_mau_value = 50
         lots_of_users = 100
         user_id = "@user:server"
 
@@ -166,9 +165,8 @@ class ClientIpStoreTestCase(unittest.HomeserverTestCase):
         active = self.get_success(self.store.user_last_seen_monthly_active(user_id))
         self.assertFalse(active)
 
+    @override_config({"limit_usage_by_mau": True, "max_mau_value": 50})
     def test_adding_monthly_active_user_when_space(self):
-        self.hs.config.limit_usage_by_mau = True
-        self.hs.config.max_mau_value = 50
         user_id = "@user:server"
         active = self.get_success(self.store.user_last_seen_monthly_active(user_id))
         self.assertFalse(active)
@@ -184,9 +182,8 @@ class ClientIpStoreTestCase(unittest.HomeserverTestCase):
         active = self.get_success(self.store.user_last_seen_monthly_active(user_id))
         self.assertTrue(active)
 
+    @override_config({"limit_usage_by_mau": True, "max_mau_value": 50})
     def test_updating_monthly_active_user_when_space(self):
-        self.hs.config.limit_usage_by_mau = True
-        self.hs.config.max_mau_value = 50
         user_id = "@user:server"
         self.get_success(self.store.register_user(user_id=user_id, password_hash=None))
 
diff --git a/tests/storage/test_monthly_active_users.py b/tests/storage/test_monthly_active_users.py
index bc53bf095..447fcb3a1 100644
--- a/tests/storage/test_monthly_active_users.py
+++ b/tests/storage/test_monthly_active_users.py
@@ -19,94 +19,106 @@ from twisted.internet import defer
 from synapse.api.constants import UserTypes
 
 from tests import unittest
+from tests.unittest import default_config, override_config
 
 FORTY_DAYS = 40 * 24 * 60 * 60
 
 
+def gen_3pids(count):
+    """Generate `count` threepids as a list."""
+    return [
+        {"medium": "email", "address": "user%i@matrix.org" % i} for i in range(count)
+    ]
+
+
 class MonthlyActiveUsersTestCase(unittest.HomeserverTestCase):
-    def make_homeserver(self, reactor, clock):
+    def default_config(self):
+        config = default_config("test")
 
-        hs = self.setup_test_homeserver()
-        self.store = hs.get_datastore()
-        hs.config.limit_usage_by_mau = True
-        hs.config.max_mau_value = 50
+        config.update({"limit_usage_by_mau": True, "max_mau_value": 50})
 
+        # apply any additional config which was specified via the override_config
+        # decorator.
+        if self._extra_config is not None:
+            config.update(self._extra_config)
+
+        return config
+
+    def prepare(self, reactor, clock, homeserver):
+        self.store = homeserver.get_datastore()
         # Advance the clock a bit
         reactor.advance(FORTY_DAYS)
 
-        return hs
-
+    @override_config({"max_mau_value": 3, "mau_limit_reserved_threepids": gen_3pids(3)})
     def test_initialise_reserved_users(self):
-        self.hs.config.max_mau_value = 5
+        threepids = self.hs.config.mau_limits_reserved_threepids
+
+        # register three users, of which two have reserved 3pids, and a third
+        # which is a support user.
         user1 = "@user1:server"
-        user1_email = "user1@matrix.org"
+        user1_email = threepids[0]["address"]
         user2 = "@user2:server"
-        user2_email = "user2@matrix.org"
+        user2_email = threepids[1]["address"]
         user3 = "@user3:server"
-        user3_email = "user3@matrix.org"
 
-        threepids = [
-            {"medium": "email", "address": user1_email},
-            {"medium": "email", "address": user2_email},
-            {"medium": "email", "address": user3_email},
-        ]
-        self.hs.config.mau_limits_reserved_threepids = threepids
-        # -1 because user3 is a support user and does not count
-        user_num = len(threepids) - 1
-
-        self.store.register_user(user_id=user1, password_hash=None)
-        self.store.register_user(user_id=user2, password_hash=None)
-        self.store.register_user(
-            user_id=user3, password_hash=None, user_type=UserTypes.SUPPORT
-        )
+        self.store.register_user(user_id=user1)
+        self.store.register_user(user_id=user2)
+        self.store.register_user(user_id=user3, user_type=UserTypes.SUPPORT)
         self.pump()
 
         now = int(self.hs.get_clock().time_msec())
         self.store.user_add_threepid(user1, "email", user1_email, now, now)
         self.store.user_add_threepid(user2, "email", user2_email, now, now)
 
+        # XXX why are we doing this here? this function is only run at startup
+        # so it is odd to re-run it here.
         self.store.db.runInteraction(
             "initialise", self.store._initialise_reserved_users, threepids
         )
         self.pump()
 
-        active_count = self.store.get_monthly_active_count()
+        # the number of users we expect will be counted against the mau limit
+        # -1 because user3 is a support user and does not count
+        user_num = len(threepids) - 1
 
-        # Test total counts, ensure user3 (support user) is not counted
-        self.assertEquals(self.get_success(active_count), user_num)
+        # Check the number of active users. Ensure user3 (support user) is not counted
+        active_count = self.get_success(self.store.get_monthly_active_count())
+        self.assertEquals(active_count, user_num)
 
-        # Test user is marked as active
+        # Test each of the registered users is marked as active
         timestamp = self.store.user_last_seen_monthly_active(user1)
         self.assertTrue(self.get_success(timestamp))
         timestamp = self.store.user_last_seen_monthly_active(user2)
         self.assertTrue(self.get_success(timestamp))
 
-        # Test that users are never removed from the db.
+        # Test that users with reserved 3pids are not removed from the MAU table
+        # XXX some of this is redundant. poking things into the config shouldn't
+        # work, and in any case it's not obvious what we expect to happen when
+        # we advance the reactor.
         self.hs.config.max_mau_value = 0
-
         self.reactor.advance(FORTY_DAYS)
         self.hs.config.max_mau_value = 5
-
         self.store.reap_monthly_active_users()
         self.pump()
 
         active_count = self.store.get_monthly_active_count()
         self.assertEquals(self.get_success(active_count), user_num)
 
-        # Test that regular users are removed from the db
+        # Add some more users and check they are counted as active
         ru_count = 2
         self.store.upsert_monthly_active_user("@ru1:server")
         self.store.upsert_monthly_active_user("@ru2:server")
         self.pump()
-
         active_count = self.store.get_monthly_active_count()
         self.assertEqual(self.get_success(active_count), user_num + ru_count)
-        self.hs.config.max_mau_value = user_num
+
+        # now run the reaper and check that the number of active users is reduced
+        # to max_mau_value
         self.store.reap_monthly_active_users()
         self.pump()
 
         active_count = self.store.get_monthly_active_count()
-        self.assertEquals(self.get_success(active_count), user_num)
+        self.assertEquals(self.get_success(active_count), 3)
 
     def test_can_insert_and_count_mau(self):
         count = self.store.get_monthly_active_count()
@@ -136,8 +148,8 @@ class MonthlyActiveUsersTestCase(unittest.HomeserverTestCase):
         result = self.store.user_last_seen_monthly_active(user_id3)
         self.assertNotEqual(self.get_success(result), 0)
 
+    @override_config({"max_mau_value": 5})
     def test_reap_monthly_active_users(self):
-        self.hs.config.max_mau_value = 5
         initial_users = 10
         for i in range(initial_users):
             self.store.upsert_monthly_active_user("@user%d:server" % i)
@@ -158,19 +170,19 @@ class MonthlyActiveUsersTestCase(unittest.HomeserverTestCase):
         count = self.store.get_monthly_active_count()
         self.assertEquals(self.get_success(count), 0)
 
+    # Note that below says mau_limit (no s), this is the name of the config
+    # value, although it gets stored on the config object as mau_limits.
+    @override_config({"max_mau_value": 5, "mau_limit_reserved_threepids": gen_3pids(5)})
     def test_reap_monthly_active_users_reserved_users(self):
         """ Tests that reaping correctly handles reaping where reserved users are
         present"""
-
-        self.hs.config.max_mau_value = 5
-        initial_users = 5
+        threepids = self.hs.config.mau_limits_reserved_threepids
+        initial_users = len(threepids)
         reserved_user_number = initial_users - 1
-        threepids = []
         for i in range(initial_users):
             user = "@user%d:server" % i
-            email = "user%d@example.com" % i
+            email = "user%d@matrix.org" % i
             self.get_success(self.store.upsert_monthly_active_user(user))
-            threepids.append({"medium": "email", "address": email})
             # Need to ensure that the most recent entries in the
             # monthly_active_users table are reserved
             now = int(self.hs.get_clock().time_msec())
@@ -182,7 +194,6 @@ class MonthlyActiveUsersTestCase(unittest.HomeserverTestCase):
                     self.store.user_add_threepid(user, "email", email, now, now)
                 )
 
-        self.hs.config.mau_limits_reserved_threepids = threepids
         self.store.db.runInteraction(
             "initialise", self.store._initialise_reserved_users, threepids
         )
@@ -279,11 +290,11 @@ class MonthlyActiveUsersTestCase(unittest.HomeserverTestCase):
         self.pump()
         self.assertEqual(self.get_success(count), 0)
 
+    # Note that the max_mau_value setting should not matter.
+    @override_config(
+        {"limit_usage_by_mau": False, "mau_stats_only": True, "max_mau_value": 1}
+    )
     def test_track_monthly_users_without_cap(self):
-        self.hs.config.limit_usage_by_mau = False
-        self.hs.config.mau_stats_only = True
-        self.hs.config.max_mau_value = 1  # should not matter
-
         count = self.store.get_monthly_active_count()
         self.assertEqual(0, self.get_success(count))
 
@@ -294,9 +305,8 @@ class MonthlyActiveUsersTestCase(unittest.HomeserverTestCase):
         count = self.store.get_monthly_active_count()
         self.assertEqual(2, self.get_success(count))
 
+    @override_config({"limit_usage_by_mau": False, "mau_stats_only": False})
     def test_no_users_when_not_tracking(self):
-        self.hs.config.limit_usage_by_mau = False
-        self.hs.config.mau_stats_only = False
         self.store.upsert_monthly_active_user = Mock()
 
         self.store.populate_monthly_active_users("@user:sever")
diff --git a/tests/test_mau.py b/tests/test_mau.py
index eb159e3ba..8a97f0998 100644
--- a/tests/test_mau.py
+++ b/tests/test_mau.py
@@ -17,47 +17,44 @@
 
 import json
 
-from mock import Mock
-
-from synapse.api.auth_blocking import AuthBlocking
 from synapse.api.constants import LoginType
 from synapse.api.errors import Codes, HttpResponseException, SynapseError
 from synapse.rest.client.v2_alpha import register, sync
 
 from tests import unittest
+from tests.unittest import override_config
+from tests.utils import default_config
 
 
 class TestMauLimit(unittest.HomeserverTestCase):
 
     servlets = [register.register_servlets, sync.register_servlets]
 
-    def make_homeserver(self, reactor, clock):
+    def default_config(self):
+        config = default_config("test")
 
-        self.hs = self.setup_test_homeserver(
-            "red", http_client=None, federation_client=Mock()
+        config.update(
+            {
+                "registrations_require_3pid": [],
+                "limit_usage_by_mau": True,
+                "max_mau_value": 2,
+                "mau_trial_days": 0,
+                "server_notices": {
+                    "system_mxid_localpart": "server",
+                    "room_name": "Test Server Notice Room",
+                },
+            }
         )
 
-        self.store = self.hs.get_datastore()
+        # apply any additional config which was specified via the override_config
+        # decorator.
+        if self._extra_config is not None:
+            config.update(self._extra_config)
 
-        self.hs.config.registrations_require_3pid = []
-        self.hs.config.enable_registration_captcha = False
-        self.hs.config.recaptcha_public_key = []
+        return config
 
-        self.hs.config.limit_usage_by_mau = True
-        self.hs.config.hs_disabled = False
-        self.hs.config.max_mau_value = 2
-        self.hs.config.server_notices_mxid = "@server:red"
-        self.hs.config.server_notices_mxid_display_name = None
-        self.hs.config.server_notices_mxid_avatar_url = None
-        self.hs.config.server_notices_room_name = "Test Server Notice Room"
-        self.hs.config.mau_trial_days = 0
-
-        # AuthBlocking reads config options during hs creation. Recreate the
-        # hs' copy of AuthBlocking after we've updated config values above
-        self.auth_blocking = AuthBlocking(self.hs)
-        self.hs.get_auth()._auth_blocking = self.auth_blocking
-
-        return self.hs
+    def prepare(self, reactor, clock, homeserver):
+        self.store = homeserver.get_datastore()
 
     def test_simple_deny_mau(self):
         # Create and sync so that the MAU counts get updated
@@ -66,6 +63,9 @@ class TestMauLimit(unittest.HomeserverTestCase):
         token2 = self.create_user("kermit2")
         self.do_sync_for_user(token2)
 
+        # check we're testing what we think we are: there should be two active users
+        self.assertEqual(self.get_success(self.store.get_monthly_active_count()), 2)
+
         # We've created and activated two users, we shouldn't be able to
         # register new users
         with self.assertRaises(SynapseError) as cm:
@@ -93,9 +93,8 @@ class TestMauLimit(unittest.HomeserverTestCase):
         token3 = self.create_user("kermit3")
         self.do_sync_for_user(token3)
 
+    @override_config({"mau_trial_days": 1})
     def test_trial_delay(self):
-        self.hs.config.mau_trial_days = 1
-
         # We should be able to register more than the limit initially
         token1 = self.create_user("kermit1")
         self.do_sync_for_user(token1)
@@ -127,8 +126,8 @@ class TestMauLimit(unittest.HomeserverTestCase):
         self.assertEqual(e.code, 403)
         self.assertEqual(e.errcode, Codes.RESOURCE_LIMIT_EXCEEDED)
 
+    @override_config({"mau_trial_days": 1})
     def test_trial_users_cant_come_back(self):
-        self.auth_blocking._mau_trial_days = 1
         self.hs.config.mau_trial_days = 1
 
         # We should be able to register more than the limit initially
@@ -176,11 +175,11 @@ class TestMauLimit(unittest.HomeserverTestCase):
         self.assertEqual(e.code, 403)
         self.assertEqual(e.errcode, Codes.RESOURCE_LIMIT_EXCEEDED)
 
+    @override_config(
+        # max_mau_value should not matter
+        {"max_mau_value": 1, "limit_usage_by_mau": False, "mau_stats_only": True}
+    )
     def test_tracked_but_not_limited(self):
-        self.auth_blocking._max_mau_value = 1  # should not matter
-        self.auth_blocking._limit_usage_by_mau = False
-        self.hs.config.mau_stats_only = True
-
         # Simply being able to create 2 users indicates that the
         # limit was not reached.
         token1 = self.create_user("kermit1")