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 Violet spaded wires (1) from the CH Overheat Thermostat and measure the Resistance across the Thermostat.
Reconnect wires after test.