cat concrete/single_pages/page_not_found.php
<?php  defined('C5_EXECUTE') or die(_("Access Denied.")); ?>

<h1 class="error"><?php echo t('Page Not Found')?></h1>

<?php echo t('No page could be found at this address.')?>

<br/>
<br/>

<a href="<?php echo DIR_REL?>/"><?php echo t('Back to Home')?></a>