I do about 12K miles per year, seen that you can make biodiesel for 9p a litre (provided you can get waste veg oil free) and its legal to make 2,500 litres a yr without being taxed on it.
Even if the engine only lasted 80k miles or something it would still save about 7grand in fuel costs! put the money you would have spent on fuel in a "just in case savings account fund" and before you know it you'll have enough money saved up for a new car anyway!