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 boiler to ignite & operate.
Does the gas Inlet pressure remain satisfactory throughout operation?
Note.
When the test is complete the pressure test point should be tightened and checked for gas soundness.