I once had a small local workshop do a small job on my car, but the guy then did a disappearing act leaving my car locked in his yard. I had a quick consultation with a local solicitor who confirmed my suspicion that this was a civil matter, which the police would not help with. He did express his opinion that the police would be unlikely to come after me if I was to break in and retrieve my car especially if I were to leave the premises secured by replacing the lock on the gate. The solicitor made it clear this was what he might do in similar circumstances and not professional advice.
Anyway, I broke in, recovered my car and never heard anything about it.