Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach pressure gauge to gas valve outlet pressure test point
Remove connector from modulating solenoid (blue wires)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Boiler On/Off switch On.
Reset lockout if necessary.
Allow boiler to commence its ignition sequence and ignite.
Observe the pressure gauge, does the burner run at low gas pressure (approximately 3.8mb)?
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.