Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Selector Switch in HOT WATER position
Connect a U-Gauge to the 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 DHW Tap fully open does the Burner ignite and operate at its maximum gas pressure of approximately 15.3 mb (+/- 0.5 mb)?
Isolate Mains Power to appliance
as per TOP's P4
Note: When U-gauge is removed pressure test point should be tightened & checked for gas soundness.