A big thanks to MattyKerby01 for his time and patience today, as well as davs4 for his help with the discs and advice.
So what have I had done?
1. New B5 RS4 discs
2. Porsche Cayenne 6-pot calipers c/w pads.
Ok so having heard about this mod a few months ago and realising the OEM brakes just aren't up to the job, I found some calipers on ebay which came with the pads and clips (maybe 75-80% pad left). After reading through various options from this thread I decided to go the easier route and have new B5 RS4 discs. I did consider AP Racing discs too which I almost went for.
Managed to get the discs machined down to 355mm dia by davs4.
Managed to get the calipers machined down by MattKerby01. After much dithering I went for 3mm, knowing more could come off if need be.
Ok offered up the caliper on the new disc and realised I'd forgotten the stepped down bolts. Bollox. No biggy says Matt I've got some tubing the right dims. What we didn't have though is bolts long enough to go through the new calipers and mounting lugs. No biggy says Matt we'll pop down to my works and get some more. A right Aladdin's cave down there.
Anyway armed with bolts, locking nuts, washers, and a 16mm drill bit, we proceeded to drill out the extg 14mm hole on the arm, to receive the new 16mm bolts to fit through the 16mm calipers holes! Easy-peesy.
After some lost skin and some swearing, and plenty of tea from Mrs Matt we eventually got the first caliper/disc fitted. Not alot of clearance between the calipers and discs, but enough. With new pads though, maybe not. Therefore probably better for 4mm to have come off the calipers, for future reference.
Managed to get the pads in and realised the restraint (spring) clip was rubbing against the disc edge. Therefore discs probably should have been turned down an extra 1-2mm.
Next up the swapping of fluid pipes. Matt had some super-duper hydraulic pump thing, which allowed you not to have to manually pump the fluid through to the calipers. POP!
Too much pressure into the fluid resevoir, and it formed a crack towards the top, spraying fluid (almost) everywhere. Oops 
Back to doing it manually. All good, but need to fit a new tank now. 
Did the same for Caliper Nr 2. Packed everything away and took the old girl (my car not Mrs Matt ) for a spin. Some god awful noises came out of the wheel arch area. Amyway all seems ok now, after the drive down the motorway.
Will take the wheels off tomorrow and give everything a good clean and maybe take some pics. And probably get the discs turned down another 1mm to stop any more rubbing issues.
All in all very pleased, and can't wait to try them out in anger
