Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove both Red/black wires from diverter valve microswitch.
Remove both Red/black wires from diverter valve microswitch.
Measurement:
DHW tap fully ON.
Measure short circuit resistance across the terminals of the diverter valve microswitch.
DHW tap fully OFF.
Measure open circuit resistance across the terminals of the diverter valve microswitch.
Reconnect wires after test.
Measure short circuit resistance across the terminals of the diverter valve microswitch.
DHW tap fully OFF.
Measure open circuit resistance across the terminals of the diverter valve microswitch.
Reconnect wires after test.
Isolate as per TOPs P4
and refit as necessary