Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach a U-gauge to the P1 test point of the gas valve
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

As the Ignition sequence starts do you see the pressure alter on the gauge?
It should show a negative pressure (approx. - 2mb) as the fan tries to pull gas through the valve.
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.