Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach pressure gauge to the inlet pressure test point
Gas On
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

On/Off switch on.
Allow fan to run and ignition sequence to commence and burner to ignite.
Does the pressure gauge show a constant inlet pressure of approx 20 mb throughout the operation?
Note:
When "U" gauge is removed the test point should be tightened and checked for gas tightness.