Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the inline connector to the DHW microswitch
Remove microswitch from diverter valve body

Measurement:

Test 1:
Measure the resistance between the two pins inside the connector to the DHW microswitch.
Expect continuity / zero resistance.
Test 2:
Gently press the switch of the microswitch and fix in the "pressed in position" using tape or a cable tie.
Expect open circuit.
Are the resistances measured as expected?