Set Appliance State:

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

Observation:

Reset appliance as necessary.
Observe U-gauge as boiler commences ignition sequence.
Does the inlet pressure remain above 17mbar (NG) throughout?
(25mb for Propane, 20mbar for Butane)
Note:
When pressure gauge is removed the pressure test point
should be tightened and checked for gas tightness.