Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Operating switch in Heating & Water position
Temperature control to Max
Connect U gauge to burner pressure test point
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Main burner ignites and remains at minimum pressure for approximately two minutes.
Does burner gas pressure increase to the set level over the next minute?
Note:
When U-gauge is removed pressure test point should be tightened & checked for gas soundness.