A similar thing happened to a friend of mine. The driver collided with him, but later tried to claim that it was her right of way (despite the fact that we were on a crossroads and the traffic lights were out of order - so there was no right of way to speak of at the time).
People are prone to lying about the facts of an accident so they can dodge liability.
If you were on the roundabout first, you have right of way.
You should ask your insurance company to launch a formal investigation and damage assessment, if they aren't already. The damage to your vehicle will prove that the girl is lying.
If you don't challenge what she is saying, then both her insurers and yours will accept her version of events (they will be happy to do this, because it'll mean that you will end up paying for the repairs to your car and not them).