I actually managed to achieve what I needed to do in the end by using the below:-
<?php if(get_post_meta($id, "h1_title_override", true) !== '') { ?>
<h1><?php title_override_text(); ?></h1>
<?php } else { ?>
<h1><?php title_text(); ?></h1>
<?php } ?>
With the necessary functions in place of course…