Hi Tamsin, thanks for your reply, big relief that I don't have to position them all by hand!
For anyone else's reference, I had the slopes modelled as 3d polys which is why the tool wasn't reading the slopes. I swapped them out for site models but then I had a problem with the plants generated by the Landscape Area Tool either not moving down at all, or moving down all at the same level when I used the send to surface command. I fixed this by making sure the shape I drew with the Landscape Area Tool was all within the boundary of the site model I was drawing on top of, then the plants corrected their heights automatically.
Thanks again,
James