Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach U-gauge to inlet pressure test point
Gas ON
Boiler on/off switch ON
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow fan to start and ignition sequence to run.
Does the U-gauge show a constant inlet pressure
of at least 18mb throughout the test?
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.