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
DHW temperature control to MAX
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow ignition sequence to commence.
Does the inlet pressure remain above 18mb throughout the ignition sequence?
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.