Set Appliance State:

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

Measurement:

Observe U-gauge, is there an inlet pressure of at least 20 mb throughout the test?
Note:
When U-gauge is removed, test point should be tightened and checked for gas soundness.