Set Appliance State:

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

Measurement:

Allow boiler to ignite does the pressure gauge show a constant gas pressure of a minimum of 18 mbar?
Note:
When pressure gauge is removed pressure test point should be tightened & checked for gas soundness.