Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Attach pressure gauge to the inlet pressure test point
Apply CH Demand
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 pressure gauge show a pressure of at least 18mb throughout the ignition sequence?
Note:
When "U" gauge is remove test point should be tightened and checked for gas tightness