Set Appliance State:

Mains Power Off
Gas Off
Turn Hot Water Tap On
Pre-heat switch OFF
DHW temperature set to maximum position
Attach U-gauge to burner pressure test point
Gas On
Mains Power On

Measurement:

Set flow rate at tap to approximately 6 ltr/min.
Allow boiler to fire at its maximum burner pressure (14.5 mbar) and water to heat.
Does burner pressure reduces as water temperature demand is met?
Note: When U-gauge is removed pressure nipple should be tightened & checked for gas soundness.