Unfortunately the hard facts are; if their is still outstanding finance on the vehicle then the finance company still have legal title to the vehicle... which they are entitled to recover.
It would then be your option to decide whether or not to pursue legal action against the seller for the return of your money.
I presume you have firstly contacted this company to ensure they are genuine, or tried talking to the seller to see if this situation is as claimed.? With hindsight a HPI check would have highlighted the true circumstances, but if you wish to check whether the claim made by the finance company is authentic... try below.
http://www.hpicheck.com/newfrontend/