Ultimate 403 Error Page

If user isn't logged in it displays login prompt, if user is logged in and still gets a 403 then it displays a 'you are not authorised' message.

Step 1 of 2

Create a page and assign in admin/settings/error-reporting

Step 2 of 2

Add the following to the page:

<?php
global $user;
if ($user->uid) {
    Print "Although you are logged in you are unfortunately not authorised to view this page.";
}
if (!$user->uid) {
    print drupal_get_form('user_login');
}
?>