after a lot of reading old threads, I thought i'd upgrade to 312 brakes, I needed new discs and pads, and thought I may as well to fill the wheels nicely. It appeared quite simple, new carriers and a new discs and pads.
I found a set on ebay of MK5 gti ones with nearly new discs, and got those. they came with everything luckily.
Firstly the discs are a different size, they are wider and have a different smaller bore.
I had to get some A6 ones which fitted perfectly.
then the carriers fitted perfectly to the calipers butthe pads wouldn't sit flat, there wasn't quite enough room to get the tabs in place. so used the MK5 calipers too which needed different lines.
as I'd painted the carriers gold, and mk5 calipers were black already my brakes are now black and gold!
anyhow just saying this for people thinking of upgrading that the mk5/s3 ones are different to the passat/audi A4/A6 ones, so watch what you buy.