The alternator is not working, it could well be electronically controlled in this case so this is where the problem could lie else I suspect a problem in the charging circuit, clearly it's not the alternator as you have replaced it with a known working unit. In the case of a Ford this is known as the "Smart Charge system". It should be charging higher than the open circuit voltage that does not neccessarily mean 14V so long as it has enough power to overcome the internal resistance of the battery, I.E 13V +.

Even if it was something draining the battery the alternator should still charge!

FletchCossie that does seem a bit low I am assuming this is with no load? If so under load you will probably find this drops further, I suspect the alt itself has gone, in winter the electrical system is under far higher demand (I suspect it was weak before) and as such this has probably taken it's toll on the alternator and finished it off.

Martin