Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Thermostat turned to maximum
Connect a U-gauge to the Burner Gas Pressure Test Point
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the boiler to execute its ignition sequence.
Is the burner pressure as per appliance data badge ?
Note:
Burner pressure should be adjusted if required.
When gauge is removed pressure test point should be tightened and checked for gas tightness.