Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap ON
DHW Temperature Control to MAX
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 if necessary (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 (28mbar for LPG appliances) at max output.
Note:
When pressure gauge is removed the test point should be tightened & checked for gas soundness.