Set Appliance State:
Gas Off
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Measurement:
Disconnect the two Yellow wires from the boiler case switch and measure the resistance across the disconnected switch terminals whilst depressing the switch button.
Reconnect wires after test.
Note:
Case switch is situated on the control panel, the control box will have to be opened to provide access to the switch.
Care should be taken when reassembling the control box.
Reconnect wires after test.
Note:
Case switch is situated on the control panel, the control box will have to be opened to provide access to the switch.
Care should be taken when reassembling the control box.
Isolate as per TOPs P4
and refit as necessary