Thanks Glenn - I have had a look through lots of S550 posts relating to water leaks, & am now starting to feel lucky !! All the ones I have found so far refer to leaks INTO the car - trunk / rear floor / front floor !!! I'll keep looking :)
I eliminated the run-off from the rear window as a culprit today - litres of water pored over it, & it all goes into the plenum outside the trunk weather seal, & runs freely out from under the tail light assemblies - nothing drops from the bottom edges of the inner guards.
I am beginning to think that it relates to a problem with the sealing of the inner guards - water is thrown around by those big tyres, & is settling into voids behind the inner guard pieces, then slowly draining out where it can. So maybe Sikkaflex over all the seams of the inner guard panels ??
Thanks Mate !