So after the front wheelarches and suspension / brakes refurbishment, the area under the windscreen and scuttle drains were next up for inspection and cleaning.
This area under the bottom windscreen trim had definitely seen better days. The trim is easy to pull off and reveals the lower edge of the glass and more access to the structural area onto which the screen is bonded. I'm not sure if there's anything between the glass and the body (such as an additional bit of trim?) but have heard tales about windscreen fitters not wanting to fit new screens due to the corrosion around the aperture onto which the glass is bonded.
I guess I'll only find that out if the screen needs replacing at some point, but have addressed the corrosion problems to hopefully stop it spreading any further and keep it under control. There's a couple of nasty holes there now which weren't present when it left the factory, so will be keeping an eye on it. The area has been cleaned back as much as is possible and treated, finishing off with a coat of Dinitrol. The rubber trim was thoroughly cleaned and sealed to the windscreen to prevent water collecting on the affected metal area below the screen. Finally the underside area has been coated with silicone grease to expel any further moisture. Once the rot is in there, the only way to get rid of it is to cut it out, so I'm hoping the cleaning and protecting will give a few more years of use out of the car.