Set Appliance State:
Gas Off
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
System Pressurised
Disconnect wires from Water Pressure Switch COM (Yellow) and NC (White)
System Pressurised
Disconnect wires from Water Pressure Switch COM (Yellow) and NC (White)
Measurement:
Measure open circuit resistance across Water Pressure Switch COM (Yellow) and NC (White).
Reconnect wires after test.
Reconnect wires after test.
Isolate as per TOPs P4
and refit as necessary