I've tried removing different items from the scene and I found that everything in the scene will render on the GPU but not all of it together. I've tried lowering the bucket size to 16 and switching off progressive render but that doesn't help. The only causes I can think of are that the scene is too big for the graphics card or there is some combination of items that conflict in some way and don't work together but I can't be bothered to go through different combinations of stuff to check this.

I suppose I just have to accept that my rather old computer is a bit limited when using the latest software. It is useable, I can render on the CPU, it just takes longer.