Worth giving all this a read. Lots of possibilities listed here, unfortunately the owner never came back with an update.

www.fordownersclub.com/forums/topic/105105-wet-pas...s

By pouring water into different areas I was able to work out that it only leaks into the footwell when I pour the water into the wing section below the bonnet hinge directly below the bottom corner of the windscreen.

Good grief, that reminds me of my 1969 Triumph 1300, which collected water in its nearside sill. Only difference was [a] it collected there until I could hear it sloshing when I braked, and [b] the car was only 3 years old, not 15. I could let the water out by removing a handy grommet.

I might guess that water thrown up from the wheels gets in near the top of the wing somewhere, so removing the arch-liner may enable you to trace the problem. You may be unable to avoid damaging the retaining studs though. If this is a long-standing problem, I wonder how long those sills may be worth protecting at 15 years of age.