So I took a look at the adminadd.php file and found the following lines :-
require_once('../login/inc_authorize.php');
require_once('../Connections/b3connect.php');
require_once('../Connections/inc_config.php');
Because $path wasn't being set until inc_config.php, this explains why inc_authorize.php was bumping me to a non existent index.html page and the reason auth was failing was because the min requirements to make an ID Level change is 2 (admin)
Moving the inc_config.php statement to the top of the includes list fixes the 'page not found' error when a user accesses a feature that they are not allowed to use.
All works nicely now :-)