Set Appliance State:

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

Measurement:

Does the pressure gauge show a constant gas pressure of a minimum of 16.5 mbar?
Note:
When pressure gauge is removed pressure point should be tightened & checked for gas soundness.