Set Appliance State:

CH Demand
Isolate Mains Power to appliance as per TOPs P4
CH/HW switch in CH position
Connect a U-gauge to the gas valve outlet pressure point
On/Off switch ON
Ensure boiler not in Pre-heat mode
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Does the U-gauge show correct working pressure and does the burner go to high flame?
Note:
When U-gauge is removed pressure test point should be tightened & checked for gas soundness.