Is it worth downsizing my car to save on petrol, road tax and insurance or just keep it?

I have a 2 litre car and it is 3 years old. It is more expensive to tax, insure and run. It cost me 10,000� to buy. Now if I want to sell, it's value is 5000� or so, 4500� for part exchange. Tax is 210� for the year, insurance 360�, petrol costs 100� per month. Service and parts expensive. Maybe a Ford Fiesta or small car like that may be better, but will I lose too much after part exchange etc for it to be worth my while?