Set Appliance State:

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

Observation:

Mains reset switch On.
Allow fan to run and ignition sequence to commence.
Does the inlet pressure remain above 18mb throughout the ignition sequence?
Note:
When pressure gauge is removed pressure test point should be tightened & checked for gas soundness.