Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach pressure gauge to gas pressure inlet 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.
Allow appliance to commence its ignition sequence.
Does the pressure gauge show an acceptable minimum gas inlet pressure of at least 17 mbar throughout the ignition sequence?

Note.
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.