Set Appliance State:

Gas On; Pilot On
Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Selector Switch OFF
Gas Control Knob in Main Burner position
Attach U-gauge to outlet pressure test point
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Does main burner ignite at 13 mbar?
Note:
When U-gauge is removed pressure test point should be tightened & checked for gas soundness.