Personally I can't see that you have done much wrong here. In my opinion (and that is all it is) I think if the left hand lane on an approach was used to take the first exit only, wouldn't this cut down on a lot of such accidents as it would stop people driving over an exit in the left hand lane where people could be exiting from the right hand (if that makes sense!). I believe this is the case here.
As you were already on the roundabout I would be giving my insurance hell if they agreed to a 50/50.
Just out of interest (as this happens to me on my way home an awful lot) please could I have an opinion on this scenario. I leave the motorway and want to exit at the 3rd exit (3/4 way round) so take the right hand lane of the slip road. I stay in this lane until just before the exit I want, begin to drift over (after checking my mirrors). However a lot of people, to jump the queue, come up the slip road in left hand lane and drive round the outside to come off the same exit. Who would be at fault if I was to drift over into them?