diff --git a/src/labthings_fastapi/testing.py b/src/labthings_fastapi/testing.py
index a16334e0..029f43c7 100644
--- a/src/labthings_fastapi/testing.py
+++ b/src/labthings_fastapi/testing.py
@@ -1,7 +1,7 @@
 """Test harnesses to help with writitng tests for things.."""
 
 from __future__ import annotations
-from collections.abc import Iterator
+from collections.abc import Iterator, Sequence
 from concurrent.futures import Future
 from contextlib import contextmanager
 from typing import (
@@ -270,3 +270,37 @@ def use_dummy_url_for() -> Iterator[None]:
     """Use the dummy URL for function in the context variable."""
     with set_url_for_context(dummy_url_for):
         yield
+
+
+def manually_connect_thing_slot(
+    host: Thing,
+    slot_name: str,
+    target: Thing | Sequence[Thing],
+) -> None:
+    """Manually connect a thing_slot.
+
+    This will accept either a single `Thing` instance or a sequence
+    of `Thing` instances. If `Mock` instances are used, note that they
+    must pass an `isinstance` test, so should use the ``spec`` argument
+    to specify the correct class for the `~lt.thing_slot` being mocked.
+    Mock instances must also provide a unique ``name`` attribute.
+
+    :param host: the `~lt.Thing` on which the slot is defined.
+    :param slot_name: the name of the `~lt.thing_slot`.
+    :param target: the `~lt.Thing` or sequence of Things it should be connected to.
+        If a sequence of multiple Thing are passed, their names are used to create a
+        mapping.
+    :raises KeyError: if multiple targets are specified, but they do not
+        have unique names.
+    """
+    if not isinstance(target, Sequence):
+        names: str | Sequence[str] = target.name
+        things = {target.name: target}
+    else:
+        names = [t.name for t in target]
+        if len(set(names)) != len(names):
+            msg = f"Thing slot targets {names} are not uniquely named."
+            raise KeyError(msg)
+        things = {t.name: t for t in target}
+    slot = getattr(host.__class__, slot_name)
+    slot.connect(host, target=names, things=things)
diff --git a/tests/test_testing.py b/tests/test_testing.py
new file mode 100644
index 00000000..2c8d5d1e
--- /dev/null
+++ b/tests/test_testing.py
@@ -0,0 +1,57 @@
+"""Test the `testing` module."""
+
+from collections.abc import Mapping
+
+import pytest
+
+import labthings_fastapi as lt
+from labthings_fastapi import testing
+
+
+class ThingA(lt.Thing):
+    """A Thing subclass that connects to ThingB and ThingC."""
+
+    friend: "ThingB" = lt.thing_slot()
+    friends: "Mapping[str, ThingC]" = lt.thing_slot()
+
+
+class ThingB(lt.Thing):
+    """A Thing subclass that connects to a ThingA,"""
+
+    friend: "ThingA" = lt.thing_slot()
+
+
+class ThingC(lt.Thing):
+    """A dummy Thing subclass."""
+
+
+@pytest.mark.parametrize("mock_slots", [True, False])
+def test_manual_connect(mock_slots, mocker):
+    """Make sure we can create and connect Things without a server."""
+    a = testing.create_thing_without_server(ThingA, mock_all_slots=mock_slots)
+    b = testing.create_thing_without_server(ThingB, mock_all_slots=mock_slots)
+    c = testing.create_thing_without_server(ThingC, mock_all_slots=mock_slots)
+
+    testing.manually_connect_thing_slot(a, "friend", b)
+    testing.manually_connect_thing_slot(a, "friends", [c])
+    testing.manually_connect_thing_slot(b, "friend", a)
+
+    assert a.friend is b
+    assert len(a.friends) == 1
+    assert a.friends["thingc"] is c
+    assert b.friend is a
+
+    mc1 = mocker.Mock(spec=ThingC)
+    mc1.name = "mock_c_1"
+    mc2 = mocker.Mock(spec=ThingC)
+    mc2.name = "mock_c_2"
+
+    testing.manually_connect_thing_slot(a, "friends", [mc1, mc2])
+    assert a.friends["mock_c_1"] is mc1
+    assert a.friends["mock_c_2"] is mc2
+
+    with pytest.raises(TypeError):
+        testing.manually_connect_thing_slot(a, "friend", mocker.Mock())
+
+    with pytest.raises(KeyError, match="are not uniquely named"):
+        testing.manually_connect_thing_slot(a, "friend", [mc1, mc1])