Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach pressure gauge to gas valve inlet pressure test point
Set temperature selector to MAX
Mains Reset Switch On
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow fan to start.
Monitor the U gauge - the inlet pressure should remain satisfactory throughout the ignition cycle and a dip should be seen during the sparking stage indicating the gas valve is opening.
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.