Thanks, brilliant answer.
I think I know roughly where I'm going now.
I'm building this from scratch doing all roles at the moment, so on a steep learning curve.
Yep, I should have mentioned the version - as it is a brand new implementation I'm on the latest version - 2.6.something (I wish it was easier to check - I'm used to phpBB with version numbers really clear)
Current plan is to use Categories to keep the courses seperate, and then Cohorts to organise and seperate the students. Then maybe groups within that - but I haven't got that far yet.
In the end the biggest 'client' will have around 2000 students across 31 cohorts / groups (others will be much smaller) so I will have to look at importing users at some point....but for now I'm doing it manually, which doesn't seem to have a sensible work flow at all; but that could just be me as I'm learning my way around.
Thanks for all the help.