I was driving down the road and noticed a bit of thin black trim flapping outside the side window. I opened the window and put my hand up towards the roof to try to re-seat the trim and whilst holding the roof around the gutter I could feel the roof assembly moving slightly against the van gutter.
When I stopped I checked and if I squeeze the lip of the roof and the van gutter hand with my fingers I can feel movement as if a rubber seal is compressing slightly - it's only a few mm at most ...
So - the big question is

Thanks for any insights!
Chris