Individual post pages custom content

Apologies if this has already been answered elsewhere, but I haven’t been able to turn up anything yet.

I’m using WP as a CMS for a website I’m working on (still locally, nothing online to show as yet!). I’m displaying posts on a page called ‘news’ (that’s all working fine) but I want to add some additional HTML to the pages where the individual posts are shown.

Basically, all I want to do is to link back to the ‘news’ page from the child pages (the individual posts). But I obviously don’t want the link to appear on the parent page (the news page).

I’ve found a (seemingly working) function to find sub-pages:

function is_subpage() {
 global $post; // load details about this page
 if ( is_page() && $post->post_parent ) { // test to see if the page has a parent
      	$parentID = $post->post_parent; // the ID of the parent is this
        return $parentID; // return the ID
	} else { // there is no parent so...
      	return false; // ...the answer to the question is false
};
};

But have not been able to get it to work – I’ve tried using the following code:

<?php if(is_subpage('news')) : ?>
<p><a href="<?php bloginfo('url') ?>/news/">Read all news stories</a></p>
<?php endif; ?>

Any help greatly appreciated!