Thanks guys,
I was thinking that I would have to merge the tables in Power Query as I have ended up doing this quite a few times. Running class and target reports then merging them is more future proof as well I think - as long as I know what fields need to be matched.
I seem to be on an iterative process of getting data from SIMS into PowerBi then finding a more efficient way of doing it e,g, using a different report in SIMS. It's fun but frustrating at the same time.
Thanks once again for the help,
Ian