That happened to me a few years ago where some one told me a van hit the side of my car.
The witness gave me the make and reg which I took to the nearest police station.
I had to present my car documents and fill out a form. The police then gave me the details of the vans owner so I could contact them. Its best to see if they will agree to get the damage fixed otherwise it means going through your insurance so may not be worthwhile in the long term financially if its just a minor damage.
I know that roundabout and this has happened to me, it's got a big white arrow on the road explaining what lane is what.