custom 404 page

Hi Im working on a CI site which has an admin folder at the same level as the application folder, We noticed today that the custom 404 page setup in the routes file was giving a status of 200 rather than 404.

Code:

$route['404_override'] = 'error/error404';

so I added 

Code:

$this->output->set_status_header('404'); 

to the controller / method, which seemed to work... but now the admin folder just loads this 404 page? 

How do I get access back to the admin folder please? Is there something I need to setup in the routes, or Have I setup the header status wrong?

Many Thanks Paul