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;
}