Did they supply the courtesy car, as part of your policy ?
Did you at any time REFUSE to give back the courtesy car when asked to do so ?
If you gave car back as soon as requested, they have no case, even if you were paid out and they let you keep the car another year, it is up to them to ask for it back.
Any expenses incurred to them is of thier doing and you cannot be held liable.
What you'll most likely find if the car wasn't worth that much is they settled knock for knock with guilty party, a lot of insurers do this as a buddy buddy arrangement, because it is cheaper than litigation, purely down to cost, NU tried to shaft me this way, but they ended up in court and had to pay not only for the car and damages but all my expenses as well for court stuff.
I would though phone Roosie on 07850671020, and just tell him as he does this sort of thing, the solicitors are excellent they use, have cases myself and others with them.

tabetha