Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Connect a U-gauge to the gas valve outlet pressure test point
Set DHW flow rate at tap to approximately 4.5-5.0 l/min
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow boiler to ignite and heat up the DHW to approximately 55 °C.
Does the burner pressure reduce as water reaches temperature and is the DHW temperature maintained at approximately 55 °C?
Note:
When U-gauge is removed pressure test point should be tightened & checked for gas soundness.