I have heard that carbon build-up can gradually affect performance so was wondering if I could get any advise on stuff you could add to fuel to help make sure this nasty stuff doesn't become a problem! Have used STP before on a DTurbo S 306 but that was the deisel version. Is there specific ones for turbo petrol cars or the most suitable for 1.8 T 20 valvers?

Have used STP petrol treatment on a few cars, the alternative is an "engine flush" which you add just before an oil change. The engine flush looks very effective, given the amount of crap that comes out with the oil ! One of the easiest ways (so I have been told) to prevent carbon build up is to give the car a good thrashing every so often !