Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
CH Demand
CH temperature control to max
Attach pressure gauge to the inlet pressure test point
ON/OFF switch ON
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow fan to run, ignition sequence to commence and burner to ignite.
Does the pressure gauge show an inlet pressure of 17mb or greater throughout the operation?
Note:
When "U" gauge is removed test point should be tightened and checked for gas tightness.