At 12.x volts with the car running and a verified good battery, the alternator is not charging. Plain and simple.
Obviously, the alternator itself was not the reason. Firstly, check the big alternator fuse, in the fuse box on top of the battery. If that's good, then look at the small wire connected to the alternator. You should have 12v there with the key on.
If you have no power on the small wire with the key on, figure out where that circuit is broken, starting with fuses. If you do have power at the small wire, ohm out the big wire between the alternator and battery.
A Bentley manual would be huge here.