Thanks for that, it works a treat for me as a dev being able to take control of the page. I'm still not sure how editor users could access it? It's less of an issue for me now though, thanks to you pointing out Security_login.ss. Also I've found some of the $lang stuff so I'm overwriting some of the default text for the page via my main _config.php.
Issue now is I'm trying to add a widget to the sidebar of /Security/login page.. but without it being in the site tree in the CMS I'm a little lost on how to do this. I might create a diff thread for this question but thought I'd ask on this one first.
Actually, I've sorted the issue regarding Widgets on the Login page, in short I've added "securityLoginWidget()" function to Page_Controller which retrieves a particular widget via DataObject::get_by_id(), and then in Security_login.ss I use <% control securityLoginWidget %> to render it on screen.
Probably a little hacky, but it's doing the trick for the time being.