In theory if the insurers get an accessor involved he/she should be able to tell from the damage who's fault it was...especially as it was on a roundabout. I.e. as you were both effectively turning right then there will be direction of impact marks on the old guys car.

His panels will be pushed towards the front of his car if he pulled in front of you or if you switched lanes and drove into him the dent will almost look as though you have t-boned him. This only works as you were travelling and turning in the same direction.

I had a simmilar situation when a woman drove into my sisters stationary morris 1000. The woman claimed my sister had opened the door into her path (which would have caused the door to be opened fully and ripped off), but in fact my sister ad only opened the door a few inches stopping to let the other car drive past and the door was driven into the a-post on a line which showed the woman was going to hit the car regardless of whether the door was open or not! I wonder what she was doing at the time...clearly not looking where she was going....

..Oh and the crazy mare

decided after crashing into my sister it would be a good idea to get out of her car and scream at my sister..

..who was in shock, distress.....oh and trapped in the drivers seat....until she'd pulled herself together to climb over and get out of the passenger side! She was very lucky to escape virtually uninjured...Phew!