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 between the exposed terminal of the thermostat to the screw securing it to the pipe.
Expect open circuit (O.L).
Is the resistance measured as expected?