After spending hours staring at the screen, I know why it's doing it, just don't know how to stop it...
It's picking up each incident of a lesson so if a pupil has 4 English lessons, they appear 4 times - rather than just once. How do I filter the lessons so there is just one line regradless of how many times the lesson appears - I've tried ticking 'Display first record only' but it just crashes SIMs.
The tree stucture is below - if it's still needed...