Set Appliance State:

Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Selector switch to Water
Connect U-gauge to pressure test point on gas valve
Run DHW boiler until water runs cold before turning mains power on
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Allow fan to run and burner to ignite.
Does the U-gauge read approximately 14.2mbar?
Note:
When U-gauge is removed pressure test point should be tightened & checked for gas soundness.