You can make claim against someone you dont hit if you can prove 'causation'. However from what you say you dont appear to have the TP (third partys) number plate so therefore WHO would you propose to claim against?? By not having TP details you would not find a solicitor in the land who would take this claim on a no win no fee basis, so therefore when you say your mate has nothing to lose in actual fact he would. Even if you find the TP you would have a major issue claiming off him, IF its his word against yours without independent evidence or CCTV footage. He could realistically just deny all knowledge and your insurance company would close the case. The last issue is about your friend not being a safe distance behind the car infront. I highly doubt the car making the manouvre did a emergency stop before u turning so whilst it may be illegal they could argue that you were not a safe distance behind. Think of it this way, If you are on a motorway and the car infront brakes to a stop(even in fast lane), if you go into the back of him or swerve and hit something then it would be classed as a FAULT accident due to you not being a safe braking distance behind. Hope this help even if its not what you probably want to hear.