Be very careful with this approach. I understand that some of these companies will provide you with a car (and they stress it can be an equivalent quality car) for no charge to you - BUT then they claim an absolutely outrageous daily hire charge from your insurer or 3rd party. The danger is that the third party claims that you are not being reasonable in your claim and it ends up in a dispute. If you can demonstrate that you reasonably need a car (to go to work for example) then you should hire a car at a reasonable price from a local or national supplier and let the third party know what you are doing and why (and the daily amount) and that you will be recaliming the costs from them as part of the claim against them. They should forward it to their insurers and this will motivate them to move more quickly to settle the claim one way or another.
Make sure you put it all in writing.
I've just had an equivalent vehicle on hire whilst mine was repaired for a no-fault prang (not shopping at Comet this Christmas....). The accident management company spoke to the insurers for the perpetrator and agreed the liek for like hire up front.