I carefully followed the walkthrough here: http://codex.wordpress.org/Child_Themes and fully copied my parent header.php in to my child theme.
The child theme showed up in my WP dashboard as expected and I was able to activate it. The customised child style.css was implemented as expected.
In my child header.php I call on three includes, found in my child theme's includes directory, using the 'include_once' method as such:
<!-- Variables -->
<?php include_once('includes/vars.php'); ?>
<!-- /Variables -->
<!-- Social menu -->
<?php include_once('includes/social-menu.php'); ?>
<!-- /Social menu -->
<!-- LiveChat Button -->
<?php include_once('includes/livechat.html'); ?>
<!-- /LiveChat Button -->
Despite this, my header fails to produce any of the three includes. I'm wondering if there's something I'm overlooking or if perhaps I've incorrectly implemented the child header.php.