Set Appliance State:

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

Measurement:

Remove both White wires from the Overheat Thermostat (thermocouple cut-out) and measure the resistance across the Thermostat.
Reconnect wires after test.