I was at a car boot sale today and was offered one of the paintings that was stolen last week in paris?

I was looking at some ornaments and the stall owner asked if I liked paintings, as he had some old ones in his boot. I recognised the painting from the paper. The chap was French so that confirmed my suspicion. What should I do, tell the police? I've emailed the Antiques Roadshow but haven't heard back as yet.