Set Appliance State:

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

Observation:

Tap reset button to skip self check.
Monitor the inlet pressure as the boiler carries out its ignition sequence.
The inlet pressure should remain above 18mbar throughout.
Notes:
Loosen inlet pressure screw several turns to obtain a good reading.
When U-gauge is removed the test point should be checked for gas tightness.