Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Gain access to inner combustion chamber
Disconnect wires from overheat thermostat
(If side access is poor the right deflector can be removed by unscrewing the three retaining screws)

Measurement:

Measure the resistance between the overheat thermostat connections.
Resistance should be short circuit.