Update:
I restored the website to an earlier state, before I modified the email template files.
I then ran a test and the default email template sent ok.
I then used the WP Dashboard to make small modifications to the email template here:
wp-admin/edit.php?post_type=wpdmpro&page=emails&task=template
But then when I tested it, no email was sent.
This system seems very fragile. Any ideas?
Thanks.