Hi again, thanks for responding!
I think that maybe I have been barking up the wrong tree as after much searching, thinking and your help, I think it is a problem with the Participants list or permissions.
My course admin role at system level has 'view courses without participation' checked. Plus for my role, system, user, category and course are also checked.
So far so good, in that users assigned here do not appear in the course participants list but under other users. I have this role and My courses block renders as it should.
However users allocated this role at category level appear in all the course participants list under all participants. I can't remove them from this list and as a consequence all the courses in the categories they are assigned too are also appearing under the My Courses block.
I have checked and they are only allocated this role at category level.