Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach FGA / U-gauge to burner pressure test point
Run the hot water tap with a flow rate of approximately 5 l/min
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Monitor the burner pressure.
As the appliance reaches the DHW temperature set on the boilers
thermostat does it begin to modulate (burner pressure reduce)?
Note:
When "U" gauge is removed test point should be tightened and checked for gas tightness.