Important we use the correct terminology here, "courtesy car" is a car provided out of courtesy whilst your vehicle is being repaired by the repairer at no charge to you. The option you are talking of is a "credit hire" vehicle. This is a vehicle provided to you on "credit" as you cannot afford to hire a vehicle yourself. It is very important that you fully read any contract you are looking at entering into. The reason for this is that the providers have to prove you could not have afforded to hire a car yourself and that your are "impecunious". Should it go to court, you may be asked to provide your bank statements to prove you did not have the funds. When you have had an accident that is not your fault, you are fully entitled to be placed in the situation you were in prior to the incident, this includes the use of an alternative vehicle. Advise any company that you want to use that you wish for them to place in writing that they "guarantee" you will not be charged in any way and they guarantee to get the full cost from the at fault insurer. Also have them place in writing to "guarantee" they will have no need for your financial details. If they do all of that, you should have no issues using them.