Set Appliance State:

Mains Power Off
Gas On; Pilot On
CH Demand
Rotary Switch in CH and DHW position
CH Thermostat adjusted to Maximum
Attach a U-gauge to the gas valve outlet pressure test point
Keep Boiler Case Switch depressed
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Does the burner crosslight from the pilot and after a few seconds operate at its set burner pressure of between 3.2-7.6mb?
Notes:
Factory heat setting is 3.2mb, the maximum setting is 7.6mb.
When U-gauge is removed pressure nipple should be tightened & checked for gas soundness.