I was always under the impression that you can't really dispute a rear end shunt, ??
You were really lucky , shunted from behind by a speeding driver , obviously taking undue care and attention of the road , Fact is he could have killed you. In my opinion he is to blame , let the insurance company sort it out , just keep it plain and simple, slowed down for roadworks , audi driver hit me from behind , end of story !
I hope your neck is not too sore if it is get it checked out and report it to the insuarnce company .
a word of caution do not discuss any accident at the scene when emotions are running high ,