tbh the £57.50 is probably not the deciding factor. If you want to try to get your kat back on the road the decision is made for you.
If you aren't going to try to get it back on the road, then if you had the car back could you get more than £57.50 for it? If so, would the amount of time/effort be worth it?
Ultimately the decision has to be yours, but if yours is a high mileage/older model and you could add a reasonable amount to the £XXXX to get a younger/lower mileage model and you wanted to have a kat I would probably take as much as I could from the insurance company and buy the younger model.
I happen to know that a puma in excellent condition is about to come on the market, too! If I could justify running 2 cars I would be tempted to buy it back myself!