I had the same thing with my Citroen C4. The consumer credit law says that you can terminate your agreement without any effect on your credit history. I don't know the full in's and out's but if you had a three year deal you can cancel and hand back the car after you have made half of the payments which in this case would be 18 months. Or if it was a two year deal 12 months.
The best was forward is to contact the finance company which for Peugeot and Citroen is usually Banque PSA and ask for a voluntary termination. They will ask for this in writing and you can do this by e-mail. About a week later you will have a call asking when they can pick the car up. Once they have done this you get another letter saying the contract has been closed.
Hope that helps.
Garry