Set Appliance State:

Gas Off
CH Demand
Isolate Mains Power to appliance as per TOPs P4
If you will not able to turn off CH demand while observing U-gauge then enable the internal programmer. Click HERE for instructions
Attach U-gauge to gas valve inlet pressure test point
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow boiler to carry out its ignition sequence and burner to ignite.
Turn off CH demand while observing the U-gauge.
Does the pressure return to standing pressure immediately after demand or is there a delay in returning to standing pressure?