Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove Front Panel of Electrical Control Box

Measurement:

Boiler Casing Switch Released
Measure open resistance across Yellow wires of Boiler Casing Switch.
Boiler Casing Switch Depressed
Measure short resistance across Yellow wires of Boiler Casing Switch.