Gav,
I'm not sure what the et is of the S6 to be able to tell you whether the RS4 wheels will fit. The easiest way is for you to take a look at one of your current wheels. It should be stated either on the wheel underneath the centre cap or on the back of one of the spokes.
The pcd on your car is 5x112, it's just whether the et 29 of the rs4 wheel will make it stick out too far.
John