As the last 2 have posted, by telling them you only paid 400 for it, means they're going to try and get away with giving you 400.
What you need to do is, find a handful of sales that match age/mileage/condition of your car, based on what people have posted above that seems to be in the region of 1000-1500. When the handler phones you back later, you need to tell her that if the car is written off, then to get another car back with a similar spec will be £X cost. Send her the details of the cars you've found. You will then insist if they're not happy to recalculate the price then you can take the car to a garage for an estimated repair bill and provide them with either option. If they're still refusing to budge, then tell them you will be complaining to the financial ombudsman - remember to make notes of the call. And raise the complain with the ombudsman (they will be able to provide some further advice).
I thought i'd read in a previous post that the insurance company have given you a courtesy car? If that's the case then good, they'll try and speed the claims process up whilst they have to pay out for rental of a car. If you don't have one yet, get that sorted with them too. By the sounds of things this isn't going to be a quick solution.
Also to reiterate the first point, stop giving them ammunition to fire at you. It's irrelevant how much you paid for the car, and even if the restoration parts were free, and you have a family mechanic who could do all the work for you, for free. None of that will help you get back to the position you were in before the crash. Only a repair bill, or a monetary sum for a similar car will.
