Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach pressure gauge to gas isolation valve test point
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance if required.
During the ignition sequence does the U-gauge show an acceptable gas inlet pressure of 20mb (N.G) or 37mb (L.P.G)
Note.
1. It may take approximately 40 seconds for the boiler to recognise
a demand and for the fan to run, this will depend upon the boiler status.
(Status code guidance CLICK HERE)
2. When pressure gauge is removed the pressure test point should be
tightened and checked for gas tightness.