Thanks ever so much Mark (moltuae) for chipping in with that bit of historic information about your similar situation. It seems like I'm afflicted with a very similar problem. Due to the fact that I've already spent £1200 approx. trying to get this fuel pump issue sorted, I'm loathe to spend even more money but it looks like the only viable option at this stage.
Had a chat with A8Parts and they have one in stock. Time for Johnny G to check his wallet and make a decision. Cheers for everyone's input.
P.S. I think my fuel pump was working all the time and was erroneously replaced.
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2010 B8 S5 Sportback quattro 3.0 Supercharged in Sprint Blue (current girlfriend)
2000 D2 A8L quattro 4.2 in Audi silver (my ex-girlfriend)