Set Appliance State:

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

Observation:

Observe U-gauge as boiler goes through its ignition sequence.
Does the pressure stay above a minimum of 18mb throughout the ignition sequence?
Note:
After this test remove the gauge and make sure all disturbed gas connections are checked for tightness.