I think I'd check things like the charging circuit and the alternator drive belt first before I splashed out on a new alternator. They're not all that cheap. Make sure all the cables are connected and have good connections, and that the drive belt is nice and tight and the thing is actually turning when the motor is running (OK, that might not be quite so easy to check now . Could even be that the rectifier diodes have blown up - in more modern cars they are often in the back of the alternator and can be replaced without replacing the whole alternator. Somewhere in there there is a voltage regulator too ....

Anyway, sounds like the alternator is having no effect at all and that the battery has been carrying the whole load, so I'd be looking for things like broken/disconnected cables first. Good luck!!

(list 6284, Mk1 S/N 00299 4GB blue [for sale]. Mk2 S/N 080000094 6GB blue)