Weirdly I tried another theme, and the path to the folder now works - although I've lost the super-extensive folder menu and icons that were there previously. Still at least I can access my folder from the dock, and it's faster than the launcher(?) I'd made.
Thanks for the help, when I've got a bit more time I'm going to read back and try and figure out what happened, as I'm not sure I've really solved the problem.
hugo7:~$ nautilus --new-window
A folder still opens though 99% sure it's nautilus ( definitely not in Caja.)
The folder opens quite slowly though. Before it was saying; Gtk-Message: 16:19:36.489: Failed to load module "topmenu-gtk-module"
...but this ( https://zxtech.wordpress.com/2018/06/15/how-to-fix-topmenu-gtk-module-error-in-ubuntu-18-04-lts/ ) fixed it.
Weirdly I tried another theme, and the path to the folder now works - although I've lost the super-extensive folder menu and icons that were there previously. Still at least I can access my folder from the dock, and it's faster than the launcher(?) I'd made.
Thanks for the help, when I've got a bit more time I'm going to read back and try and figure out what happened, as I'm not sure I've really solved the problem.