Firstly, I assume the other side admitted liability?
Who's insurance did the claim go through, yours or his?
If he admitted liability and it went through his insurance, I would want to know why you are now being chased?
There are industry standard payout structures e.g they will pay a body shop between £30 and £60 per hour or £20 per day for a hire car etc. I would assume that the other guys insurance will have agreed to pay out a set figure and the accident management company were pressing for more (to cover the costs of your hire car), the case probably went to court and the accident management company probably lost, leaving them with a deficit to recover.
If this is the case, I would want to know WHO is actually chasing you for the money?