I sorted this problem by adding <br class="clearBoth" />, as below:

<br class="clearBoth" />
<?php
require($template->get_template_dir('tpl_modules_xsell_products.php', DIR_WS_TEMPLATE, $current_page_base,'templates'). '/' . 'tpl_modules_xsell_products.php');
?>

The <br class="clearBoth" /> causes the DIV to break completely and start on a new line below, very handy...