Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Boiler ON/OFF switch OFF
Attach U-gauge to gas valve outlet pressure test point
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Boiler ON/OFF switch ON.
Allow boiler to commence its ignition sequence.
Does U-gauge show a positive gas pressure?
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.