custom menu not showing in post of categories

I’ve been racking my head for days and can’t seem to get this sorted.

I have a custom menu called ‘x’ it contains categories and single posts, it’s location is set to ‘single-x.php’
when I view the category archive from menu-x the menu is present, but when I view single posts from category-x (or it’s children) it is not.

In my index.php I have:

} if ( in_category('x') ) {
    include (TEMPLATEPATH.'/single-about.php');

But no luck for single post views.

I have also tried adding:

} if ( in_category('x') || post_is_in_descendant_category( 'x' ) ) {
    include (TEMPLATEPATH.'/single-about.php');

Along with this in my function.php to help with the posts of descendant categories of ‘category-x’:

if ( ! function_exists( 'post_is_in_descendant_category' ) ) {
	function post_is_in_descendant_category( $cats, $_post = null ) {
		foreach ( (array) $cats as $cat ) {
			// get_term_children() accepts integer ID only
			$descendants = get_term_children( (int) $cat, 'category' );
			if ( $descendants && in_category( $descendants, $_post ) )
				return true;
		}
		return false;
	}
}

Still no luck in single post view!
Any help is greatly appreciated!!!!