Set Appliance State:
Mains Power Off
Set Multimeter to Resistance 
Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Measurement:
Remove the diverter valve microswitch cover and disconnect the microswitch multiplug. Measure the resistance across the grey wire and pink wire on the 3 wire diverter valve microswitch.
Isolate as per TOPs P4
and refit as necessary