Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach U-gauge to outlet pressure test point on gas valve
DHW temperature selector set to Mid position
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Boiler ON/OFF switch ON.
Allow boiler to heat DHW to approximately 55 degrees C, reduce flow rate at tap to assist temperature rise if necessary.
Does burner pressure reduce as demand is met?
Note:
When U-gauge is removed pressure test point should be tightened & checked for gas tightness.