Sorry to hear about your predicament. The bench mark ruling applied in such cases is referred to as National Employers v Jones
http://www.car-crime.com/n_e_m_.htm Which basically says that you can never buy true title to stolen goods. So despite how many hands the bike has passed through since it's theft it still belongs to the original loser. That title then passes to the insurance company if there is a pay out. That said you entered into a contract with the person who sold you the vehicle and you do have redress against them. They in turn have redress against the person they bought from, so on and so forth, down the chain. So if what you are saying is true a main dealer may end up paying out to one of the parties. It can get really messy. As for HPI, did you check both the VRM and the VIN when you did the check? HPI will try and squirm out of paying out so be robust. Also, insurance companies are approachable and will negotiate.
If you want any further info send me a U2U and I will explain why I have knowledge of the above.
[Edited on 8-3-2006 by chrisw]