Collision on roundabout was not my fault but witness is elusive

I was recently involved in a car accident on a roundabout. I was on the inside lane to come off the 3rd exit. As I passed the first exit a van came on the roundabout and hit my left side (bit of the front door, wheel arch and all of front left side panel. His company is denying liability and saying I changed lanes, which I didn't. I have a witness which should help with my case. My insurance company informed me they have left messages with no reply, however, I managed to get in contact with the number he gave me. He wouldn't give me his address but have asked to send the form to his local post office which he will collect. His reaction to my case does not give me much faith that he will be helpful. Is there anyway that the police can get involved and used their power to get a statement? But the witness seems to be the type that does not want too much involvement. I heard a lot of these roundabout cases settles for 50/50, however, I feel this is really unfair when I was clearly not at fault. Please advise.