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
Unclip Diverter Valve head from the body and leave
in a safe position
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).
Operate boiler at maximum output. CLICK HERE FOR INSTRUCTIONS
Does the pressure gauge show a constant inlet gas pressure above 16.5mbar (28mbar for LPG appliances)?
Note:
When pressure gauge is removed the test point should be tightened & checked for gas soundness.