Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Ensure DHW taps are OFF

Measurement:

Disconnect the flying lead of the flow switch at the inline spade connector that is situated in the middle of the lead that runs from the flow switch to the PCB.
Measure resistance across the leads that run to the flow switch.
Reconnect leads after test.