Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
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.
Turn on the hot tap, 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.