Set Appliance State:
Gas Off
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Disconnect the Red wires from the HW Flow Sensor
Disconnect the Red wires from the HW Flow Sensor
Measurement:
Measure the resistance across the Sensor.
The resistance measured across the Sensor should be steady or only slowly changing.
The resistance will vary depending boiler temperature and should be approximately 64 K ohms at 10 degrees C and 3 K ohms at 80 degrees C.
The lower the water temperature the higher the resistance.
Replace leads after test.
The resistance measured across the Sensor should be steady or only slowly changing.
The resistance will vary depending boiler temperature and should be approximately 64 K ohms at 10 degrees C and 3 K ohms at 80 degrees C.
The lower the water temperature the higher the resistance.
Replace leads after test.
Isolate as per TOPs P4
and refit as necessary