Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect one of the black wires from the safety thermostat
located on the flow pipe, top right hand side of the appliance

Measurement:

Measure resistance across the exposed terminals of the thermostat.
Expected resistance closed circuit (zero Ohms plus resistance of test leads).