Don't think I explained that too well - apologies!
Basically, when I assign 'Manager' at a category level, I'd really like them to be able to 'manage' everything in their category. That would ideally include being able to delete sub-categories.
But having spent a bit more time digging, it seems this is a known bug - at least according to comment by Andrea Bicciolo on this discussion:
On the tracker, there's also this earlier stuff:
So I think I'll just wait until it's fixed - unless there's already a patch/fix available?
Thanks again for help, Mary.