How do I override the default admin page permissions?

[Jonty Needham]

Currently I want to allow certain users access to a certain administrator page to allow or deny access for other users for a part of the tool.

I have overridden has_permission to grant access, however it says that "You don't have permission to edit anything".

I've since tried overriding has_add/delete/change_permission in the model_admin to true. but that's not helped the issue.

Anyone know how to do this?

I'd rather not create another group.