Set Appliance State:

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

Measurement:

Appliance On/Off switch On.
Reset lockout as necessary.
Allow fan to run and ignition sequence to commence.
Does the gauge show a constant inlet pressure of at least 18 mb throughout the sequence?
Note:
When pressure gauge removed test point should be tightened and checked for soundness.