Set Appliance State:

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

Measurement:

Allow the boiler to light, ensure the inlet pressure remains above 18mb.
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.