If you did not report it stolen to your insurance company, as you said you didn't want to claim and just wanted it back. Then that may well be the stumbling block, as when the police run it through their computor it will come back as being legally owned, registered and insured by the new owner/keeper. Therefore it becomes a civil matter as to who has legal title to it.
Have you at anytime received a letter from dvla informing you that someone has appiled for a V5 for it. Or is the car now on different number plates.