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 pressure gauge show an acceptable gas inlet pressure (minimum 16.5 mbar for natural gas, 28mbar for LPG)?
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.