Just thought I'd give an update on this.
My S3 is now at 7000miles, and I did my usual mainly motorway run, which I havent done in maybe 3000 miles, I used to get between 26 and 28mpg average over the whole of this run (~90 miles each way, probably 80 of those miles on the motorway ), yesterday I got 32mpg on the way there and 34mpg on the way back.
I've no idea why the sudden increase in MPG, I'm very surprised by it. the speeds were broadly the same, I didnt drive differently to normal, it was shell v power in the tank, which is what I use all the time. Not sure whether its to do with the engine loosening up, or the warmer whether , or both, but I'm really surprised at the improvement. Anyway its all good.