Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach pressure gauge to 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:

Reset appliance (press and hold reset button for 5 seconds, then release).
Allow the boiler to commence ignition sequence.
Does the pressure gauge show a constant inlet gas pressure.
(Minimum mbar requirement at gas valve inlet 16.5mbar at max output)
Note:
When pressure gauge is removed the test point should be tightened & checked for gas soundness.