Set Appliance State:

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

Observation:

Allow burner to ignite and establish and boiler to run for DHW demand.
Slowly turn down the DHW temperature control while monitoring the U-gauge.
Does the boiler modulate the burner pressure as it reaches target temperature?
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.