Im sorry, but what they told you is 100% correct, it IS up to the Finance Company to log their interest in a vehicle, and a HPi report (or any other report) does not garauntee you that a vehicle does not have outstanding finance on it.

However, if you bought the vehicle in good faith as a private individual (rather than as a Company) to a degree the law is there to protect you and all may not be lost. Seek legal advice if you are in this situation.