I had something very similar. The 3rd party insurer started to complain and refused to pay the hire care charges (which were crazy high). It seems that there are some agreed rates for the whole industry which are so high but the 3rd party should pay. Mine had a court date and I was told that I would need to attend and provide any supporting evidence and this was a condition of the original agreement I signed with the CMC. I also had signed some insurance thing too to cover any costs. In my case the 3rd party settled the charges before the court date as they knew they were standard!! (according to my insurance company).
I only had a little 307 thing as a replacement for my 325 (they wanted to get me something bigger) and the costs for a couple of weeks were 1000s