Worked a treat.
For others – if you add that, it appears on every page, even with comments switched off.
I copied my page.php file, renamed to commentspage.php and put this at the top:
<?php
/*
Template Name: COMMENTSON
*/
?>
Then I uploaded, and edited the COMMENTSON template to include the <?php comments_template(); ?>
Then on my page where I wanted comments, I changed the template (Manage, Pages) to the COMMENTSON template.