Set Appliance State:

Mains Power Off
Gas On; Pilot On
CH Demand
Selector Switch ON
Gas Control Knob in Main Burner position
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the burner to ignite.
Run the heating to build up temperature inside the boiler
Slowly turn the potentiometer down towards the minimum position while observing the U-Gauge
Does the gas pressure decrease as the burner modulates ?
Note:
When U-gauge is removed pressure test point should be tightened and checked for gas soundness.