Set Appliance State:

Gas Off
On/Off Switch OFF
CH Demand
Isolate Mains Power to appliance as per TOPs P4
Attach pressure gauge to inlet pressure test point
External controls calling for heat
CH temperature switch to maximum
DHW temperature switch to minimum
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Turn On/Off switch to On.
Allow ignition sequence to commence and sparking to occur.
Does the pressure gauge show a constant inlet pressure of at least 18 mb throughout the sequence?
Note: When pressure gauge is removed pressure test point should be tightened & checked for gas soundness.