Set Appliance State:

Gas Off
Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Mode Selector Switch to DHW position
NOTE: Remove Boiler Flow Microswitch Cover (depress single rear plastic clip) to view
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Does the Boiler Flow Switch spindle operate the Boiler Flow Microswitch?
Replace Microswitch Cover after test.
Note:
This test is for the lower microswitch.