id say alternator...
if it is pay the £150 or whatever they want to change it, cos on the rt its an absolute b**ch to do, from experience, and the parts at halfords are going to be about £80 + the deposit for taking one back.
its unlikely to be the belt slipping but it could be... if it is the alt it will just get worse until it leaves you starnded, remember that when that red light is on its not charging the battery and so you are running out of time!