Set Appliance State:

CH Demand
Isolate Mains Power to appliance as per TOPs P4
Boiler On/Off Switch ON
CH Mode Switch in its 24hrs position
Connect a U-Gauge to the Gas Valve outlet pressure test point
CH Temperature selector on maximum
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

When Boiler has reached a reasonable operating temperature turn down the CH temperature selector slowly.
Does the Burner gas pressure reduce with Boiler temperature and CH temp selector (CH Potentiometer demand, see (1))?
Notes:
It may be necessary, to reduce the heating load by turning off some radiators, to reduce time taken for this test.
When U-gauge is removed pressure test point should be tightened & checked for gas soundness.