If it's a fake, then you are the victim. Minichamps have no right to request the bike be sent to them, it's your property to do with as you choose.
If it is a fake, then you have a right to make a criminal claim against the seller, as do Minichamps, but if you honestly no longer have the sellers details other than that from ebay, you can't give them anything more.
It's not an offense to buy or own a fake, but it is an offense to produce and sell fake goods (all sorts of standards, copyright rules etc).
I'd be keeping very tight hold of the bike, if they want it back, I'm inclined to think there's more to it than it being a potential fake, if it was they'd just say, "it's a fake, where did you buy it" and that'd be it.
I agree with Adam that you just keep a tight hold and let Minichamps do the rest. I'd just send them this link and say, I got it from here...
http://cgi.ebay.co.uk/ws/eBayISAPI.d...LanguageToggle [Edited on 2-5-2005 by Mr_S]