All I can say is keep phoning phoning phoning the insurance company who is paying out on this for you. Ring at least once a day, as you are only entitled to the hire car for a finite number of days and will be left without transport (unless you kick up a stink).
Find out what the going rate for your written off car would be (ask the garage it's in at the moment, they should have a trade book listing prices of cars by year and mileage etc) and insist you get that amount. They may offer you the price less 1 or 2k, as you could get this much from scrappage of your written off car. If you are getting no answers, keep going up the chain until you hit the highest manager you can. You will have to keep on their case as it can take weeks to get things sorted, and TBH in my experience (and anecdotally that of friends) the threat of involving your solicitor is the only thing that will get things moving along quicker.