Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Ensure there are no demands from External Controls
Ensure no taps are running
Remove control box cover as necessary
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Measure the voltage at the external controls into the boiler.
There should be no voltage from the external controls and no taps should be running i.e. no demands that could cause the boiler to fire.