Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach a U-gauge to the gas valve burner pressure test point
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow boiler to heat DHW to approximately 55 degrees C, reduce flow rate
at tap to approximately 5 l/min.
Does burner pressure reduce to maintain a DHW temperature of approximately 55 degrees C?
Note:
When U-gauge is removed pressure test point should be tightened & checked for gas soundness.