It is a bunch of other buttons in a grid, I think I haven't quite understood how UI elements work. It's like this:
Control:
Grid with some buttons
Control node that holds a bunch of Line2Ds that draw a grid (no control units inside of it)
Control node that holds a bunch of Line2Ds that draw a grid (no control units inside of it)
Another Grid that holds more buttons
A small panel with a final button at the bottom
I've got it sort of working at the minute. As it turns out, the two control nodes holding only Line2Ds were blocking clicks on the grid, and it's working if I put the grid below them in the tree.
It never occurred to me as there are no buttons or anything, just an empty control node which was positioned in a way that let me use anchors to get points to set out the grid.
Thanks for pointing it out, though I did feel it was a bit unintuitive that one control node would block another even when there was nothing in it, though I guess it makes sense.