Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach a pressure gauge to the burner pressure test point
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow burner to ignite and establish.
Monitor the burner pressure as boiler operates for CH.
Does it remain on maximum throughout?
Notes:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.