Just to let you all know that I got a replacement battery from the dealer for £104(inc) and fitted it this afternoon.
The only thing that I had to reset was the clock (but strangely not the date?). No error codes, warning lights or anything.
I've added a guide for replacing the battery because it's not quite as simple as other cars. Well not for me anyway!
http://www.e90post.com/forums/showthread.php?t=292200
Thanks
Steve