Set Appliance State:
On/Off Switch OFF
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove front control facia from the boiler
Disconnect all leads connection plugs from the top of the control box noting their positions
Gain access to PCB by removing two screws that hold the control box to its support panel and lifting the box clear
Disconnect the On/Off switch plug (red leads) from the PCB
Remove front control facia from the boiler
Disconnect all leads connection plugs from the top of the control box noting their positions
Gain access to PCB by removing two screws that hold the control box to its support panel and lifting the box clear
Disconnect the On/Off switch plug (red leads) from the PCB
Measurement:
On/Off switch to the On position.
Measure resistance across the terminals of the switch.
Reconnect leads after test.
Measure resistance across the terminals of the switch.
Reconnect leads after test.
Isolate as per TOPs P4
and refit as necessary