[MickyP]

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?

[olduser]

It is an offence to stop on a roundabout without good reason, exit blocked is a very good reason.

Your description suggests you were not paying attention and driving too close putting the fault clearly in your court.
I cannot think of a defence only a lesson - under all circumstances we as drivers must allow for a vehicle needing to stop, for traffic conditions, or illness, or simply mechanical failure, and must drive with that in mind at all times. Which means space to give yourself reaction time and stopping time.

[TC1474]

Originally Posted by MickyP

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.