Sorry to hear of your misfortune Richard.
I work for one of the UK's largest insurers and I am also involved in a very lengthy claim of my own at the moment that's lasted three years so far.
I can say that as long as you have an independent witness that states that the other driver's actions directly resulted in your accident, you have a good chance.
However, if neither of you caught the plate, as somebody else pointed out, it can be argued that you can't prove it was him, it could have been a visitor that just happened to have the same car.
My opinion is that you have a chance but a small one. It is worth following up though, can't hurt to try.
Keep on at the police until they give you the results of their enquiries, don't be afraid to badger them though understand of course that they have much else to do so don't expect miracles. They generally do a very good job I think, you just need a little patience.
Also, write down as many details as you can about EVERY aspect of the incident, from time, weather conditions, light, location, road conditions, road lighting etc. If it ever goes to court to be argued, having this written down now will be considered reliable but information remembered at a later date, less so. You could even get a sattelite photo of the junction from Google Earth or Microsoft Maps, it may prove useful.
Fingers crossed, hope it goes well for you.
Oh and regarding car hire, get advice from your insurer. You should always give all the information you can to your insurer - they are interested in winning claims, not losing them so they are definitely fighting your side, not his. The more you tell them, the better chance they have of giving you good advice.
Of course there are bad insurers as well as good, hopefully yours is one of the better ones!
Good luck
