Set Appliance State:

Mains Power Off
Turn Hot Water Tap On
Appliance DHW thermostat to MIN
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Turn hot water tap OFF.
When the tap is turned off the DHW demand LED should no longer be illuminated.
Does the DHW demand LED go off when the tap is turned off?
Note:
You may wish to repeat this test several times to confirm there is not an intermittent fault.