I asked our legal advisor, Lucy Bonham Carter, who replied:

“This seems to be a very minor repair and unless it makes the vehicle unsafe or unusable he should not have been put in a hire car save for the actual period when the repair was actually taking place. The chances are that everyone is seeking to run up costs on the claim. A charge may well be being made by the dealer for "storage". The problem is that if costs are being racked up unreasonably the third party insurer will probably refuse to pay because their "losses have not been mitigated". Depending on the hire agreement and the arrangements (if any) with an accident manager this could leave your son with the bill. If we are to assist on this we will need to know the names of all the companies he is dealing with - accident manager, credit hire company, garage. It would also help to know the name of the third party insurer (as some are not signatories to agreed credit hire arrangements and will oppose the claim in any event). I would also need to know the precise nature of the damage and whether this indeed does render the vehicle unsafe or unroadworthy and why the necessary part is so seemingly difficult to obtain.”