Thanks a lot for your help Fou-Lu, but after playing around with your answer I'm still quite confused.
Say my include is this;
And my page I want to include it on is this;
<li><a href="landing.php">new search</a></li>
I'm still getting a file path of 'gallery/gallery/gallery.php' if I'm on that page. This will however work perfectly when included on '(no subdir)/home.php'.
<?php require_once __DIR__ . '/../header.php'; ?>
<p>more page content</p>
I realise that's because I'm not doing what you told me properly , but could you point me in the right direction?