Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach pressure gauge to gas valve pressure outlet test point
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance if required (rotate selector switch to
'O' position, then rotate back to DHW/CH position).
Allow self-check to complete and appliance to commence its ignition sequence.
Is the gas inlet pressure at least 20mb (37mb for LPG) throughout the boiler operation?
Note:
When pressure gauge is removed the test point should be tightened & checked for gas soundness.