Hi Paul
Dani has just posted a file https://totara.community/file.php/208/moddata/forum/31/5632/Messages_and_Notifications.docx that describes all of the notifications and emails sent by Totara and how to edit these .
I think the message you are referring to is the signup and the text of the message is in the file moodle.php and it comes from the site administrator.
Does this look the same as the text you are receiving?
Craig