Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
CH Demand
Disconnect the flow switch at the inline connector
Make sure there is a system circulation path, (i.e. TRVs, bypass open)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Apply a demand and check to see if flow switch continuity can be measured across the two black wires at the flow switch connector.