Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Selector Switch to Hot Water and Heating
Set DHW temperature selector to Max
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:

Allow fan to run and ignition sequence to commence.
The inlet pressure should remain above 17mb (37mb for LPG) throughout the ignition sequence and there should be a noticeable drop as sparking occurs.
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.