Ok, I'm playing through it again.
This time I built 2 defense trees on the center planet, and it said "yay well done, etc."
Bit of a silly bug...
It only lets you complete that level if you have built some defense trees.
And if you haven't, you can't even remove some of your dyson trees and grow defense trees in their place, as there is no facility to do this.
Once you've locked your trees in, that's it.