Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Heating Switch OFF
Remove top wires from Overheat thermostat
(situated on pipe, to the left of the control box)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Observe the appliance does it now operate correctly?