i did this once with a non fault accident
its basically a vehicle hire company,the insurance or more probably the repairer set this up for him as they get kickbacks.
the vehicle was taken to the cab office and presented as a replacement vehicle,tested and plated,when the repairs are done on your motor its then represented as a replacement vehicle.
its a real pain in the arse,and the third party are not responsible for the costs,if the claim fails for whatever reason the costs will fall on the hirer

that will be you.
they supplied a brand new tx4 and the cost were somit like £270 per day,i had it for a month,and we wonder why our insurance has gone up
and yes they do try and hide the fact that its a hire to you.
you could end up out of pocket depending on your LA'S test standards when it comes to putting your motor back on road
be wary and get the repairs chased up on your own motor asap