Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove both orange wires from DHW demand microswitch
Boiler switch in standby position

Measurement:

Hot Water tap OFF.
Resistance across outer terminals of microswitch = open circuit.
Hot water tap ON.
Resistance across outer terminals of microswitch = short circuit.
Note:
The centre terminal on Switch is unused.