Set Appliance State:

Mains Power Off
Gas Off
DHW Demand
DHW Selector in Maximum (+) position
CH Selector in Minimum (-) position
ON/OFF Switch in ON position
Attach U-gauge to gas valve inlet pressure test point
Gas On
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow boiler to light and establish while observing the U-gauge.
Once the boiler has lit does the U-gauge show a constant inlet pressure of at least 18mb?
Note: When U-gauge is removed pressure nipple should be tightened & checked for gas soundness.