Got the car back from Owen Ferry Automotive a couple of days ago and, fingers crossed, all seems well!
They have a gadget that measures the temperature of the discs and Owen had phoned, after they'd taken it for a run, to say that one of the
rear discs was hotter than the other three and he was going to investigate. Earlier in this thread, someone said that you can't always tell whether the problem is front or rear brakes.
Now, that's 2 things I like about a garage already: keeps you informed and investigates (doesn't just replace parts).
So, as the photos below clearly illustrate, one of the rear calipers had the pads stuck (the rusty ones at the top of the photo) and these were catching on the disc (see the grooves at the base of 2 of the pads).
Another thing I like about a garage: they keep the old parts to show you what the problem was.
Owen phoned again to let me know the score and recommended that he replaced the rear pads and skimmed the discs. They have another gadget that does this on the hub to compensate for any 'hub run out' (I think that means it doesn't spin in a perfect circle). He also suggested that I should consider having the front discs skimmed too.
Point 4 that I like about a garage: ask the customer before racking up a big bill.
So, I had all 4 discs skimmed and replacement rear pads. Owen took the photo of one of the front discs after he'd skimmed it (another point for Owen!

) and sent it to me.
Now, I know I have to wait to see if this resolves the problems that were recurring at around 3,000 miles, but I have never had such beautifully smooth brakes - the effect of the skimming is amazing! I drove David's S8 last weekend and thought how nice his brakes were (before mine were fixed), but mine are even better now!
So, full marks for Owen Ferry: nice guy; clean, well equipped garage; brilliant customer service; cares about what he does!
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2002 Final Edition S8. Ebony black with Silver Grey leather and myrtle wood trim.
Current mods: solar sun roof, 20mm rear spacers, 15 mm on front, red brembo callipers, 6k headlights, rear view camera, engine remap, alloy dash dial rings, alloy navi rings, tt/phaeton pedal upgrade, (and custom matching foot rest) dension ipod interface & parrot hands free kit (both fully hidden), av input, tv in motion switched thro' PF switch in blanking plug right of steering column, Audi 'quattro' sill covers, repositioned centre console switches, radio clock, .
Planned mods: auto-dimming rear view mirror, dash cam (as steamship's), fit the ski hatch, refit philips drl's (or maybe not - nope, definitely not - horrible botch!).