Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach U gauge to gas valve inlet pressure test point
Unclip Diverter Valve head from the body and leave in a safe location
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Operate boiler in maximum output service mode.
FOR INSTRUCTIONS ON SERVICE MODE CLICK HERE
Does the inlet pressure remain constant and above a minimum of at least 16.5mb?
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.