Set Appliance State:

Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
CH/HW switch in HW position
Connect a U-Gauge to the gas valve outlet pressure point
On/Off switch ON
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Does the U-gauge indicate correct working pressure and does burner go to high flame?
Note:
When U-gauge is removed pressure test point should be tightened & checked for gas soundness.