I worked for vw commercials for 5 years, the lower roller on T4's was a common problem, esspecially with builder vans cos they let the roller channel fill up with crap and dust etc, but the fact that the door fell off sounds like you've been ignoring the problem for a while, whooops!!!. Matt is pretty much spot on, about 90 quid to fix, but there are cheaper options, I'd suggest ebay. Once you have fixed the problem, keep the channel and roller clean and regular use of a good spray grease should keep it problem free for years.