[mylesdw]

Simone: The figures you quote, 12, 13 etc would they be volts rather than amps, I am guessing they are?

First the ammeter that you have fitted, if it is working and wired correctly it should behave as follows:
1. Without the engine running, turn on the headlights, the ammeter should show around 10 amps discharge. Switching ANY accessory on should move the needle slightly towards discharge.
2. After starting the engine, the ammeter should show a charge of 10-20 amps for a few minutes, and then fall back to zero.
3. With the engine running, switching any accessory on may make the needle flick but it should return immediatly to zero (or very slightly positive).

If the ammeter behaves as above, it is working OK and it will show a continuous discharge if indeed the charging system is not working. Try going out for a drive with fans and lights on, if the ammeter stays at zero, the system is working.