I had a minor brainwave and remembered a mate had some leftover 40mm HDPE protective pipe duct on her farm, so I scooted over, cut a short length and set-to customising a length to do the job.
I hacksawed a split down its length, used the heat gun to soften and shape it a little and fitted it as per the photo. I made sure the split ran along at the 9 o'clock position when fitting i.e. not on the bottom, in the unlikely event any further water issues recurred. No clips were necessary.
Took the rear offside wheel and arch liner off and saw that someone had already had a half-hearted effort of putting a few globs of household silicone in the exposed hole. Cleaned everything, applied Dynax liberally and left the car on the jack until tomorrow when I'll fit and seal a plate to cover the "hole".
I'd not come across this issue at all with any of the cars nor on the forum. Definitely a poor piece of engineering design but not hard to rectify. Thanks as always for the input.
Before & after pics.
