Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach U-gauge to gas valve outlet pressure test point
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Boiler ON/OFF switch ON.
Allow boiler to commence its ignition sequence and ignite.
Observe the U-gauge.
Does it show a pressure of at least 0.7mbar?
Does the pressure increase before boiler cuts out?