Have traversed that familiar path before Tigg.

Tunnies suggestion about the wiper spindle holes is well made. I would also check the grommet in the main part of the scuttle which allows the feed pipe for the driver’s side washer nozzle to pass through.
On my car water was getting past this in into the rain deflector (foam pad) and thence into No 4 plug well - although there were breaches in the welding on the plastic seams on the outer edge of the deflector at several points and around the spindle holes.
The deflector was soaked and I ended up removing the scuttle entirely to get the thing dried out properly as it holds a surprising amount of water.
I was also surprised to find that the rubber trim that locates on the bottom of the windscreen had been dislodged from its seat thereby allowing water to migrate past it onto the back edge of the deflector probably adding to the problem.
I considered replacing the entire scuttle (with deflector) but when I approached the dealer the server allowed himself the merest hint of a smile before telling me that the part was no longer available.
Anyway I dried out the deflector, sealed up what needed to be sealed - paying attention to that grommet – and have been problem free for several weeks now.
I’ll post up some pics of what I found later on.
Windscreen trim moved upwards to show debris behind it (had moved off its seating on the windscreen edge for some reason).

View of bulkhead and scuttle drain well showing water migrating away from the well towards the middle of the scuttle (by way of the rain deflector).

The deflector seams had breached at several points on the outer edges and around the wiper spindle holes (how, is unknown).


