Thanks for the reply and suggestions Colin, sadly though I'm not really getting any further forward.
The role of Designer was just the word Teacher renamed in the Moodle language packs and not a custom role at all. I tried reverting the language name back to defaults in the forlorn hope that some bug was causing the role issue but no luck.
I also tried removing all roles except student roles from a course to see if that helped but again I just get the same issue.
I also tried manually editing the backup file and removing any non student role assignments and restoring a new zipped version but again no joy.
As the site administrator there seems no setting to grant me to Allow Role Assignments either unfortunately.
To be honest its not the student enrolments I care about so much as their activity completions and certificate awards which is why I want to duplicate each course three times and then delete the redundant activities in each one to make them smaller and more modular. I would then add a meta course above each three for future global enrolments leaving the current users intact.