Set Appliance State:

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

Observation:

Allow the boiler to fire and run for up to two minutes, observe the gauge does the inlet pressure remain above 18.5mb continuously as the appliance runs.
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.