Set Appliance State:

Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Selector switch in OFF position
CH temperature selector to minimum
DHW temperature selector to maximum
Attach U-gauge to gas valve outlet pressure test point
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

With the hot water tap on, turn the selector switch to the water heating position.
Does the burner crosslight from the pilot?
After approximately 3 to 4 seconds, does the burner go to maximum burner pressure (approximately 8.1mb)?
Note:
When the U-gauge is removed, ensure the pressure test point is checked for soundness.