Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the plastic cover from the pump proving
microswitch and disconnect wires

Measurement:

Measure the resistance across the pump proving microswitch terminals.
Note:
Pulling the microswitch out of its housing may make this
test easier if the boiler is not in a convenient location.