Thank you Martin for your patience.
The button was always there, very obvious and very functional.
My apologies for wasting your time.

OK, so you are using the Explorer interface. So the double-clicking does not work. Anyway, the toolbar button is there.
The one on the very right, next to the address bar.
https://winscp.net/eng/docs/ui_explorer

Apologies Martin my request was not clear as to what I meant.

The window I was refering to was the one you get from
View/Go to/Open Directory/Bookmark...

The button is there (with "opened folder" image).
You can also double click the path label above the panel (if using Commander interface).

The mouse movement for getting to the open directory window is quite clumsy.

Could we please have an icon on the toolbar icon which displays the open directory window.

Postioned to the left of the parent directory icon would be excellent !