Looking at mine it looks as if the plastic mushield has done it's job, I thought that the water ingress was probably from the front. If you think about the car driving down a motorway in the rain that area is right in the firing line. Although the bowl is sealed to the outer retaining panel and inner valence water, muck, salt etc must creep in from around the outer panel and just pool there, then over time the inevitable happens.
I wondered if a drain hole at the bottom of the eyebrow would help, just where it meets the inner valence.
Just a thought!
Yes but the nasty little afterthought of a plastic mudshield behind the headlight (which is only on post 76 cars) just transfers the rot point from the front of the wing to 4" back! Admittedly, that area of the wing is easier to repair than the front corner above the eyebrow - but still not ideal! A nice drain hole in the valance (it's not the eyebrow here) below the outer headlight can't hurt, I think I will incorporate that in mine!
Thinking about the seal that is the root of the OP, I can't actually see what good it is, since there is water and maybe mud on both sides of it and on later cars with the tacky mudshield, water on at least one side and probably both. And with plastic headlamp bowls, it's purpose is even more moot.
I reckon what is needed is a full arch shield, some decent drainage around the lights, the facility to easily remove the headlamp support panel with lamps to clean behind and finally lots of lovely Hammerite or similar coating both sides of the inner front panel, eyebrow etc. This is what I am going to aim for on mine, which will effectively be a daily driver so it'll need all the help it can get. I certainly don't want to be putting ANOTHER front end on it in 10 years time, i'll be over 70 by then!