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