Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
ON/OFF Switch ON
Summer/Winter button set to Winter
CH Temperature selector to minimum
DHW Temperature selector to maximum
Attach U-gauge to inlet pressure test point
Gas ON
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

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 the pressure test point should be tightened and checked for gas tightness.