Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the flow switch at the inline connector
Test to be carried on the connector leads that run to the flow switch
Note:
Flow switch is shown removed for illustration only

Measurement:

Test 1.
Measure the resistance of the flow switch at the inline connector. Resistance should be open circuit.
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On.
Reset boiler if required.
CH demand.
Test 2.
Repeat test. Resistance should be closed circuit.