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.