It's a little known fact but his insurance cannot write your car off as "uneconomical to repair". You are entitled to have your car repaired to the standard it was in before the crash without detriment to yourself. This is a statute of UK civil law and non negotiable by the insurance companies.

They can only write it off if it unsafe to repair - i.e. major chassis damage etc and then they should offer you market value for it.

If however you are happy to take the money, ignore everything I have just written above - just don't take their final offer and do some research of how much cars are selling for in the same nick as yours was before the accident.