This bloody happened to me this morning!!!
Managed to get car home... just... called BMW ES and within 40 mins a mech was at my house. Checked it out and confirmed it as alternator. It was on the back of a truck no more than 1 1/2 hours later and i've just received a BMW 3 series hire car for 1 week.
Got to say i'm well impressed with the speed and quality of the service.
Scary how many alternators are failing though.