Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Control Thermostat in high position
High/Low Switch in the Low position
Connect a U-Gauge to the gas valve burner pressure test point
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

After the Boiler has sequenced its purge period (approximately 10 seconds) does the U-Gauge indicate a positive burner gas pressure?
Note.
When U-gauge is removed pressure test point should be tightened & checked for gas soundness.