Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Ensure the boiler is not at its full operating temperature
Ensure the boiler is not at its full operating temperature
Measurement:
Remove both of the spaded yellow wires from the DHW control thermostat, and measure the resistance across the thermostat.
The DHW control thermostat is the uppermost of the two thermostats situated below the secondary heat exchanger.
Replace leads after test.
The DHW control thermostat is the uppermost of the two thermostats situated below the secondary heat exchanger.
Replace leads after test.
Isolate as per TOPs P4
and refit as necessary