If the dealer is an honourable operation, they will have checked the provenance of the car before taking it into their stock. Ask them to show you the results of their checks and take a copy, should they agree. If they wont show you the checks, or if you're not happy with their findings, walk away.

If you buy a car from a dealer in good faith and the car subsequently appears to have outstanding HP or Conditional Sale finance on it, then you would be protected under the "innocent purchaser" clause of the Consumer Credit Act. This would not, however, be the case if the car was the subject of a leasing agreement with outstanding finance.