What I think you need is this, Camshaft/Crankshaft position sensor.
Audi | Euro Car Parts UK’s No.1 Car Parts Retailer
I wouldn't trust the ones on eBay, you better off going to TPS or a dealer for the genuine part. Not cheap, but worth it.
Having said that, the one on ECP looks ok.
The Audi part number I used is :06A 906 433L
There is some confusion, as the part name seems to change from place to place!
But this is what I had changed and it sorted the problem. I hope it does for you mate.
You may want to double check the part number for the S3, it maybe different?
Do you know which one I need from the link below?
genuine audi s3 camshaft sensor | eBay
Thanks Rog, really helpful.
Takin it easy