http://www.shedstore.co.uk/

I have been thinking of these, thanks for the link. However, I am
feeling that with these I might have to do a whole lot of silicone
sealing to get the level of sealing I want. I would also have to add
some kind of rubber seal on the door perimeter, like that soft
elastomer you find around car doors, for example.

I'm really looking to find whether some manufacturer that makes a
specialist, almost watertight, shed design.