The staring point was searching the web and some careful thinking.... the problem was the joint / seam between bus and plastic roof.
I didn't fix it first time around but with a bit of patience I got it water tight.
First I de-rusted the Gutters. Then I black Hammerited them (yes I know not very original.
Next I then used caravan sealant (Sikaflex 512). I'm not sure if it is the best it just had the word "Caravan" metal & plastic on it.
The first time around it didn't work but then I released that I'd left a little lip in some places so the water was gutting under the sealant and back in the van.
Careful checking, I spotted the issues and now we're dry at night... an nolonger a Cross ing Continent.
(Whilst spread the love mastic I also made sure the pop rivets I've got in the tailgate channel at the back were also covered).
It was a bit of a roll coaster as it's not nice water pissing into your bus .... but a bit of patience and focus saved the day.... onto the next challenge.
I've got no real skills in vehicle maintenance and I'm sure competent folk would scorn my bodge job ... but it worked.