John is right, the alternators output should be the value on the voltmeter when the engine is running.
A good example of this was yesterday when my TF had yet again another flat battery after standing for 6 days. Not enough to flash the lights when unlocking it. As soon as the engine was jump started, everything was spot on, even with a flat battery - the alternator was doing all the work.
I would be looking at either the alternator or voltage regulator, then for badly corroded connections on the output or main fuse. After that check all the earth points on the vehicle.
Final check would be of course a new battery, just in case it is dragging the alternator down a bit, but to be honest, if there was a cell down you would be looking at a dead battery anyway.
Let us know how you go on...
Sundance