Set Appliance State:

Gas Off
Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Ensure that the Boiler is not in an overheat condition

Measurement:

Disconnect the two Grey wires from the Overheat Thermostat and measure the resistance across the Thermostat.
Replace leads after test.
Note:
The Overheat Thermostat may have opened and may require resetting manually.