Set Appliance State:

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

Observation:

Boiler On/Off switch On.
Reset lockout if necessary.
Allow fan to run and ignition sequence to commence.
Does the inlet pressure stay constant with a pressure of at least 16.5mb?
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.