Set Appliance State:
Mains Power Off
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Ensure that the Boiler has been running and that the Boiler is at a reasonable operating temperature.
Ensure that the Boiler has been running and that the Boiler is at a reasonable operating temperature.
Measurement:
Turn the Thermostat down to its minimum setting.
Remove both the Red/White and the Grey spaded
wires from the Thermostat.
Measure the resistance across the two spade terminals
on the Thermostat.
Remove both the Red/White and the Grey spaded
wires from the Thermostat.
Measure the resistance across the two spade terminals
on the Thermostat.
Isolate as per TOPs P4
and refit as necessary