OK, I have something sensible for future forum users and then something I found jaw dropping because I really wasn't expecting it.
First, thanks Paul. All info I found suggested I'd need to drop/lower both subframes, which seems a bit... excessive for changing cats.
The guy helping me found these
http://www.audirevolution.net/videos/86/b6-a4-3.0-downpipe-removal-part-1
http://www.audirevolution.net/videos/85/b6-a4-3.0-downpipe-removal-part-2
and I can say that it's a real f**ker getting into that top bolt to get the downpipe off.
And now to the really weird but positive result.... I had my full Milltek and S8 cats fitted in 2011, and at the same time the precats were gutted and welded up again. Then I bought a set of used resonated centre pipes almost immediately and had them fitted.
So it's a ******* miracle that my mate shone a torch into the centre pipe on the offending side and uttered, "err. come and have a look at this." We took the whole exhaust off, turned the centre pipes vertical and gave them a quick bang on the ground. This nugget fell out in 2 pieces at our feet.
It's not much smaller than the diameter of the pipe so god knows how it hasn't caused major problems until now. It must have been jammed somehow but still let gases past it until I revved the engine one day (when someone asked me if it was a diesel and I felt I should demonstrate that my car does NOT sound like a diesel), at which point it must have moved and blocked the pipe. It's a good job the blockage was after the point where the centre pipes are linked (the Y piece) because it meant that part of the gas flow could go out through the other side.
This picture shows the unresonated pipes but you can see the Y section where the gases can pass from one side to the other.
And these are the resonated pipes
The cats are 100% intact and look good as new internally so I know that nothing has passed from my existing cats into the centre pipes. So the chunk must have been in there for a long time.
The car feels 100% better now and I think I must have regained 20bhp so a good result. Plus I didn't have to buy new cats 3 years after the last pair.
So if there's a lesson in all of this it might be to check any used exhaust parts you buy and possibly give them a good thump standing vertically, just in case. I would also recommend that if you have cat problems with your original parts, just get someone to remove them, open and empty the pre-cats and then cut off the old cats and weld on a pair of decent generic replacements. Or if you can get your hands on a pair of decent cats from another car like I did then you may even be able to screw the oxygen/lambda sensors in where they are supposed to be.
