Ouch, running a S3 (or 2.0TFSI 190) on supermarket fuel. There's also Momentum 99 petrol on some/most tesco filling stations which would suit the car better. I've only ever used supermarket fuel in emergencies on my 1.0 / 1.5 normal engine let alone a high performance motor. On the S3 I believe you can actually feel the difference between 95 and 98/99 RON fuels. Some tests show even a significant bhp increase with higher RON fuel. My experience on both 1.0TSI and a 1.5T (on a Volvo V40), Esso/Shell/Jet/BP/Texaco 95RON fuel were indistiguishable from each other peformance/economy wise. Esso 97 (at the time) gave me smoothest engine, Vpower 99 gave me more mpg (but not enough to make it cheaper to use it all the time). I didn't really notice any more power, but then again even if I have 5bhp more on either engine and without a specialist timing gear or dyno it would not really be noticeable to me. In an S3 or similar, you may be able to feel the difference.
Actually remember in my Volvo V40 the fuel grade recommended was 95-98RON. 95 was for normal driving, and 98 was for 'hot weather' and 'maximum performance and economy' as stated in the manual.