Set Appliance State:

Mains Power Off
CH Demand
Pre-heat switch OFF
CH temperature set to maximum position
Attach U-gauge to burner pressure test point
Gas On
Mains Power On

Observation:

Allow fan to run and burner to ignite.
Does burner pressure reach approximately 7.8 mbar and flow pipe and radiators heat correctly?
Note: When U-gauge is removed pressure nipple should be tightened & checked for gas soundness.