Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach U gauge to gas inlet pressure test point
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

If required reset boiler.
Allow ignition sequence to commence.
Does the gas Inlet pressure remain satisfactory during ignition?
Note.
When the test is complete the pressure test point should be tightened and checked for gas soundness.