Hi,
Just a quick update...i managed to fix this today.
After some continuity testing, it ended up being as Ian suggests...a crack in a wire in the bumpers' PDC loom - mine also didn't have any visible damage similar to his RS6.
I ended up soldering a new wire connection between the PDC sensor connector and the main loom connector. Sealed the loom all up again, and jobs a goodun'!
Hope it helps someone else...
Oli