Struggling with permissions

[bobbinalong]

I'm running 1.6.09 and struggling with permissions having run happily for years with just admins. We now want specific users to be able to upload to specific albums.
Imagine I have two categories - Vehicles and Other and within vehicles, two categories Cars and Boats and then Cars has albums called Ford, Volkswagen, Citroen etc. I ideally want the Ford-admin user to be able to upload only to the Ford album and the VW-admin to the Volkswagen album but I can't see how to do that.
So I created sub-categories of Cars called Ford-cat and VW-cat and put the Ford album in Ford-cat and the VW album in VW-cat.

Then I created a group called Ford-admin and put my new user in it. Gave them Public Upload Allowed as it says in the documentation. All my albums are set to not allow public upload except Ford and Volkswagen where Visitors can upload is set to Yes.

But when I edit my Ford-cat, I can't remove the VW-admin group from the "Groups allowed to create albums in this category" box. Both groups have access when I want just one. And likewise the VW_cat. So both can upload to either category. How do I fix it?