I've had this problem on two occasions now. I believe it all started when a loose connection to the alternator regulator caused it to overcharge, and subsequently boiled the battery. Fixed this problem, battery stayed as it was as it seemed ok. Went to start the car the next day after finishing work...nothing. The battery was dead. So with a new battery fitted (with the alternator problem now rectified) the car has been trouble free and starts on the key first time.
However, upon leaving college today (or trying to whichever way you look at it) the battery was once again dead. I don't think anything was left on, so I can only imagine that the alternator was playing up again and was constantly back feeding? Had to bump start it, but was fine all the way home. My volt meter read around 14.2v when running.
The only thing I can say is that for the past couple of mornings, it has run very lumpy (like a one-legged Mr Blobby).