Dan
I've done a bit of research on this and it looks promising.
There are a couple of sites which sell alloys and explain the various configurations. I looked at
www.alloywheelsdirect.net which confirm that both th A6 and S6 in C6 guise use the same wheel set up with a PCD (pitch circle diameter apparently) of 5 X 112 with a 57mm centre bore. This would make sense since the bore then increases to 66.5mm on later A6's (why they don't fit my car now

)
One you've had a chance to look yourself give me a shout.
I will be getting the tyres changed onto wheels which fit at the weekend.
Cheers
Rich