When it was clear to do so I drove onto the large busy roundabout, a car in front and to my left was exiting the round about suddenly stopped on the round about because the exit road was busy and I crashed into the back left.
I understand it is an Interchange & a offence to stop on a roundabout but the car had no where to go, but then neither did I..
who is at fault?
You will be held liable I am afraid, it is called strict liability unless you can prove that you were brake tested or that the third party did something that can be attributed to direct causation such as in cash for crash rear enders.
In this case it seems you were distracted which means that you were not driving in a manner which allowed you to stop in the distance you could see to be clear, hence the reason you rear ended the other vehicle.
You are of course allowed to stop on the roundabout if you are prevented from proceeding (what else can you do?) as @
olduser has stated, this is one case where I am afraid you will have to take it on the chin and accept liability.