I have a Rover Mini Cooper, and the battery keeps going flat. The fan belt is fine and turns the alternator when the engine is idle but I noticed the charge getting lower until I was unable to start it. Does this indicate a problem with the alternator or fan belt, or is it symptomatic of something else?
It is almost certainly either the alternator or the battery itself. If the dashboard charge light is on then it is likely to be the alternator, otherwise get the battery checked (preferably not by the RAC judging by recent media coverage!)
If you could get it to a garage they will bring out a little gadget and test it in 30 seconds to tell you which one it is. They won't charge you (but most likely offer to fix whichever one it is!)