Hi Graham, it's not clear to me which parts you're trying to boolean together.

Are you trying to cut the upright wavy "Q" walls with a curve?

If you're trying to divide it in half by a line, you may need to move the line up in Z away from the middle of the shape. That's because a straight line right in the middle of the shape kind of has 2 directions that it can possibly cut the shape in. If you move it a little distance away from the shape, it will clear up which direction the cut should go.

But I'm not sure if that is what you are trying to do there.

- Michael