Isolate Mains Power to appliance as per TOPs P4
Remove plastic cover from diverter valve microswitch

Observation:

Turn on hot tap fully.
Does diverter valve spindle rise fully towards the microswitch?
Turn off hot tap.
Does diverter valve spindle retract releasing microswitch?
Note:
The spindle operation can be observed from the underneath the boiler. If the boiler is fitted at low level a mirror will aid observation.