OK so as I couldn't quite get my head around why the concealed gasket/trim would suddenly start to leak and having confirmed my drains were unblocked, I decided to attempt a seal repair before forking out on a replacement. Regardless of the interior seal, if more water enters the sunroof tray than the drains can cope with, then it will overflow the sliders and drop in. I confirmed this by putting a raw hosepipe on the roof just behind the sunroof with it closed. In fact it may as well have not been closed as the water funneled in, but from above the inner headlining. So I cleaned and masked off the sunroof opening with a 7day masking tape and then rubbed beeswax on it. I then masked off along the highest point of the weather seal but not all the way to the glass's as to prevent a raised lip forming that may stop any water from running away. I used tech7 polyurethane sealant for its adhesion low shrinkage and UV resistance. Once I ran a 5mm bead around the gap, I used a small flat mixing spatula to create a flat profile from the center of the weather seal to the adjacent roof edge.
After 2hrs I removed the inner tape and left the outer until the next day {24hrs}. It took a little bit of trimming with a scalpel and a bit of persuasion to break the seal and slide the glass back to remove the outer tape. I then waxed the edges again where the tape had been to prevent sticking to the tacky outer skin of the freshly cured sealant
It has absolutely worked a treat with virtually zero water ingress and it has significantly reduced the wind noise. The tighter seal will also prevent crud that could previously enter the tray and block the drains from entering the system. I would strongly suggest anyone with this problem to give it a whirl or replace the weather seal before going to the trouble of removing any fittings or trim, although in some cases that may still be necessary.