Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
The water heating gas valve is the upper of the two gas valves
Attach a U-gauge to the inlet pressure test point
Gas On

Observation:

The inlet pressure should be approximately 20 mbar.
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.