Roof Leak Mk6

[MiloMK6]

Need help resolving a leaking roof on my mk6 transit lwb 2004.

I've tried to seal it by going over the seam with Sixaflex but it hasn't fixed it. I'm unsure if it's coming in through the roof seam or from the gulley along the outside of the van?

If I need to remove all silicone and original from manufacturer then replace I will but would prefer confirmation on this?

Has anyone seen this leak before? It ends up pooling on the inside then running down into the cab or into the over head storage area.

Will attach photos, thank for the help

[Altransit]

My Mk6 Medium Roof used to leak like a sieve from the joint across the roof at the rear of the cab/front of the load area, seat belts used to get soaking wet!

I scraped all of the filler out of it and ran a bead of Sikkaflex (NOT silicon) across it leaving the fill 'proud' like on the Mk7, (which doesn't seem to leak there)