Crittall greenhouse door seal and gutter waterbutt diversion kit

Hi all. I’d just like to say how fab this site is: I had no idea what my 1980s greenhouse was and was so grateful to see people had hoarded instructions so I think I can guess that mine is a Crittall - no idea which type though!

Anyway, can anyone help me by telling me the type of gasket I need for my door frame and how to remove & replace it? I can’t seem to figure it out from the instructions which someone posted: it seems to gloss over it? I’ve attached photos: I have no idea ho to remove the flat and flap sections of the door seal?

Also, has anyone found a suitable gutter waterbutt diversion kit? I think the channel width is about 16mm?

And another pic (sorry - it wouldn’t let me upload >1 at a time)