The flat is in both my girlfriends name and my own, doesn't seem fair to put debt for my car onto that.
I guess it comes down to what your objectives are and how they are best achieved:
*Is it an investment? If so, don't drive it, and keep it in a heated garage (not a lot of fun in that)
*Is it for fun? If so, how much more fun is it than the cars you already have, and would you be better off just hiring one as and when or joining one of those car clubs?
*Is it a pride of ownership thing? Can't really help you with that one.
*As others have said, what other things could you do with the money that might prove more satisfying.
Just my random thoughts...