Not sure what the problem is...
 

... or even if this is the right section, but I've noticed a distinct change in sound of my exhaust and my fuel consumption has got worse quite suddenly.

There doesn't seem to be a rattle from the cats and there's no warning lights.

A silencer box collapsed perhaps?

Any ideas folks?

Hmm..I'm not sure if a problem with the silencer would make much difference to the fuel consumption, but I'm no expert. I hope it turns out to be something simple/inexpensive tho'

You cannot really diagnose that kind of thing over the internet and without hearing it. Could be a number of reasons including like you say collapsed silencer. Other reasons could be dodgy sensors like lambda or MAF, injector, spark plug, coil , timing issue or it could be just a dodgy batch of fuel. I think first you should check if there are any fault codes stored and look at measuring blocks for misfires, lambda trim and ignition retard. If all seems good perhaps try fresh fuel and good old Italian tune up.

Did a VCDS scan and, apart from an alarm code (I set if off accidentally yesterday) and an intermittent fault on the rear headlight level sensor (which I knew about), nothing.

So I took the 'vanity cover' off and discovered this (see attached photo).

Could have been worse, I suppose. I'll see if I can free the flap and get a new link in the week.

Edit: the diaphragm moves freely; is there something else I should check for the cause of the broken link?

I would expect that broken link to affect performance either at lower or higher end of the acceleration band depending on which one Jim, but not by the amount you're indicating in your initial post. But best to get it sorted and see if that cures it before anything else I guess

Yes, I think the higher end performance, 3K rpm plus wasn't all it should be. The arm attached to the diaphragm was a bit stiff, but it freed up easily enough. Should it be dismantled and lubricated, or should I just replace the arm now it's free?

Oh, and what's it called (or does anyone have a part number)?

In an ideal world you need to get the ends of the flaps in the manifold de-gunked and that will help them to stay free again for longer. There is a procedure for this on audi pages - I'll look for the link for you.
I had James (Mkoneman) do this to my D2 before I went to Germany in it and it drove so much better afterwards and has been fine ever since +++

Here's the link - it was on quattroworld

http://forums.quattroworld.com/a8/msgs/44689.phtml