tried to start my car today and the battery was as flat as a pancake, wouldn't even power the central locking. Now my first thought was dodgy alternator, but any time I've had problems with an alternator I've heard the car struggle to turn over days before the battery dies but yesterday everything was hunky dorey! No lights, radio or anything else left on so I haven't a clue what drained it, but if it is the alternator, how easy is it to replace these in the Marea (1.8)?