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:

Cancel purge cycle & If required, reset boiler.
Allow ignition sequence to commence and boiler to ignite and ramp up for DHW demand.
Does the gas Inlet pressure remain satisfactory throughout (see note)?