Set Appliance State:
Gas On; Pilot On
Set Multimeter to Resistance 
CH Demand
Isolate Mains Power to appliance as per TOPs P4
Remove Yellow wire (COM) and Grey wire (NO) from water pressure switch, making a note of their location
System Pressurised
Remove Yellow wire (COM) and Grey wire (NO) from water pressure switch, making a note of their location
System Pressurised
Measurement:
Measure short circuit resistance across Water Pressure Switch COM (Yellow) and NO (Grey) connections.
Reconnect wires.
Reconnect wires.
Isolate as per TOPs P4
and refit as necessary