Set Appliance State:

Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Attach U-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

Measurement:

Allow boiler to commence its ignition sequence and ignite.
Observe U-gauge, does burner run at low gas pressure (approximately 1.8mb)?
Note:
When U-gauge is removed pressure test point should be tightened & checked for gas soundness.