Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach pressure gauge to the 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 as necessary (hold reset button for 5 seconds then release).
Allow boiler to go through its ignition sequence.
Does the pressure gauge show an acceptable gas inlet pressure (minimum 16.5 mbar for natural gas, 28mbar for LPG)?
Note:
1. boiler may enter lockout during test, reset as required.
2. When pressure gauge is removed pressure point should be tightened & checked for gas soundness.