Mini roundabout accident

Hi, was wondering I could get some opinions on a recent car accident I was involved in

I was approaching a mini roundabout, my intention was to turn right, as I stopped at the give way line there was no one coming from my right (the side I'm suppose to give way to), as soon as I entered the roundabout there was a car coming really fast (40-50 mph), I was about half way around the roundabout until he crashed into my side of car (drivers side) he mainly damaged the rear end of my car. He had enough room to go around me or could have slowed down to wait until I finished my manoeuvre which was nearly complete.

I ran my insurance company up and explained this to them, that the other car crashed in to me while I had nearly established my turn onto the roundabout to the road that I wanted to go on. The insurance company straight away claim that I'm liable. However I find this hard to believe as the car was not there when I stopped to give way to anyone on my right, the car turned up while I was half way across the mini roundabout and crashed into me.

I can't see myself liable for this, if anyone has any advice please feel free to share