Ok, I love this sort of conversation re spending lots on a car to save money (executive summary to my ramblings below - dont bother)....
First, you say you worry about the mk3 breaking down- has it done this much? Any car can break down, a well maintained one stands a better chance, but newer with more electronics and sensors can mean the opportunity for more expensive misshaps where genuine parts are the only option. Bear in mind that new or old, peace of mind costs less than £100 a year in the form of a home recovery breakdown service.
You say your 21 and on your 7th car, so are you really gonna keep a Leon for 5 years?
You want to save money on fuel, by giving more of your money away in DEPRECIATION and INTEREST on the loan. This will cost far more than the fuel saving, I did a quick sum a while back, and over 8k miles, 35mpg petrol vs 50mpg deisel makes the deisel a £500 cheaper option. So the depreciation and interest will easily be more than this over even 6 months.
Why give your money away when you can enjoy burning it yourself?
You must learn to enjoy burning petrol. its great. If your living at home and can afford the fuel, why waste your money on a loan. Buy a Leon or whatever in a few years when they are 2k, and enjoy a multitude of other 10-15yr old cars in the meantime! All for less outlay that a loan on a depreciating newer car.
I think most people spend most of their disposable income on car finance, because they have nothing better to spend it on. This is great for me (currently running a £500 banger), as I can run cheap cars and spend all my money on mountain biking (equipment and travel etc). You could be aiming your surplus at somethign else, or saving for deposit/year abroad etc.