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:

Reset boiler if required.
Allow fan to run and ignition sequence to commence.
During the sparking phase of the ignition sequence the pressure gauge should show a slight drop of 1 - 2mb.
Note:
When "U" gauge is removed the test point should be tightened and checked for gas tightness.