Template_redirect for child custom post types

All sorted. For any one interested, I simply added in a function (into functions.php) that redirected to my new template dependent on the name of the post (you could also use the ID).

add_filter('template_include', 'new_template');

 function new_template($template){
       global $post;
       if($post->post_name == 'mortgage'){
           $template = get_template_directory() . '/mortgage-template.php';
       }
       return $template;
 }