Set Appliance State:

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.