Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Boiler On/Off Switch ON
Boiler On/Off Switch ON
Measurement:
Remove the Brown and Orange spaded wires on the Boiler On/Off Switch.
With the switch in the ON position measure resistance across the exposed terminals on the switch.
Reconnect wires after test.
With the switch in the ON position measure resistance across the exposed terminals on the switch.
Reconnect wires after test.
Isolate as per TOPs P4
and refit as necessary