Right, here we go!
If I get a hose and run it down the windscreen on the passenger side like this:

Then go inside and go to the passenger footwell, and look in the direction of the arrow:

Unclip the block connectors from the chassis, and look in behind, toward the front arch, you can see a black seam, where the panels butt each other. Right on the tip of the arrow:

Here is a closer view:

Then closer still, you can see the water beads all over the place:

If I look at this while water is washing down the windscreen, I can see water oozing through the black sealer and rolling down to the floor. I am going to take the front wheel and arch cover off, and tackle it from under the arch because I cannot get access from inside.