Hi, the fuel pressure reading seem correct according to this page:
http://workshop-manuals.com/audi/a8_...ing/page_5971/
I did think it may have been some sort of exhaust leak but the emission results from the last MOT came back perfect so I think the welds were okay. It was also a bit quieter after the flex pipes were repaired and no noise seems to have returned since.
As the car has 2 fuel pumps I haven't yet ruled out a fuelling problem. I think there is a main pump and a secondary pump. The main pump runs at all times, and the second pump runs for the first 30 seconds and then only at high load, although this is sometimes disputed depending on where you read and some people say both pumps run at all times.
I don't fully understand why a failing fuel pump would cause low MPG as you'd assume it would actually be the opposite - maybe a low fuel pressure could cause an inefficient spray pattern on the injectors.
Thanks