OK, I've clarified just what I need to achieve...
I'm working on a site that has a certain page (Staff Directory) that is hidden from the general public. However authorised/registered users will be able to log into the site and see this page (in the frontend of the site). However they won't be able to access the backend at all. This means I can't use the usual domain/admin access that takes you to the backend of the site.
I can't see how to do this. Does anybody know of any module, etc that achieves this?