Well, I have had everything off under the N/S front arch, and sealed all the seams before waxoiling it. I waxoiled the whole underneath the van whilst I was at it. Took the van outside yesterday evening and left it for the night. Woke up this morning and 1/2 a pint of water on the floor -- couldn't believe it, nearly had a nervous breakdown, I felt like I had just been buggered with a splintered fencepost.
Started running water down the windscreen again, and as Belly mentioned, right in the corner at the base of the windscreen there was balls of water bubbling through every 15 seconds:

That shiny speck at the tip of the arrow in the picture above, is water shining. Up closer:

Right in the corner, it is. Went outside and it coincides with these wires going into the windscreen:

Above is just for perspective. Just putting pictures for other peoples reference in case they have to go through this nightmare. Anyway, up closer:

On the last picture you can see that black sealer which is supposed to seal the wires, well it doesn't! Underneath that sealer there is some sort of white plastic retainer, well I can see this from both trhe inside and outside. I reckon if I pushed a wire in, it would poke out the other side.
Well anyway, I am now waiting for the sealer to dry which I have pushed up under the screen and around the wiring terminals, and am now going to try the water treatment and pray it is fixed once and for all. The water was flowing down the side of the screen, pooling up around this wiring terminal and flowing back in under the screen and down behind the dash, right in the corner.