Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach U gauge to inlet pressure test point
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Measure the inlet pressure as the boiler attempts to ignite.
Does the pressure remain above 18mb throughout the ignition process.
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.