Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Mains water OFF
Boiler discharged and drained

Measurement:

Remove DHW flow switch head and brass valve (this is flow switch farthest to the left of the pump and is connected by brown wires).
Check for free movement of the brass valve.
Check for open circuit resistance across switch with the brass valve removed.