Car sitting at the correct height, no apparent air leaks from any corner. It turns out my air compressor has failed, or rather working intermittently. They are replacing it + an associated relay. Both parts have apparently been superseded, so possibly a common problem or at least one waiting to happen.
I must say, Aylesbury Audi have been fantastic so far. On both occasions, they arranged a loaner car for me at short notice and kept me in the loop at all times. They're even doing my diff oils service FOC for the inconvenience, something I asked them to do whilst the car was still with them and was fully happy to pay for it.
