OK I am going to explain this very simply. Let's take your situation:
You have £1000 savings. You have £1750 overdraft used, £250 available. Your total cash available is: £1250. At any time you like, you can spend, at most, £1250. If you want to buy a car, you have £1250 to spend.
Let's say you pay off your overdraft off with your savings. DO NOT REDUCE YOUR OVERDRAFT AMOUNT. You have £0 savings. You have £750 overdraft used, £1250 available... recognise that number? Yes, you still have £1250 to spend. If you want to buy a car, you have £1250 to spend. The difference is, each month that you don't spend that £1250, instead of being charged interest on £1750, you are being charged interest on £750. The interest you are charged on that extra £1000 is MORE than the interest you are gaining from having it in savings.
So, please, pay your overdraft off, and do not reduce your overdraft limit. You will have the same amount of cash available, but will be losing less money each month on interest charges. The purchase of a car is a completely separate issue to where you should be saving/borrowing money.
This guy speaks the truth!