There are indeed such tools:

https://superuser.com/questions/1088286/how-to-restrict-mouse-to-current-display

However I'm not sure I need them. With the displays arranged not just diagonally, but pulled apart slightly, it seems I can't get the mouse to move off the main screen anyway.

The answer in that SU question suggests if you move the mouse quickly it will make it to the second screen, but I've not managed it.

Problem solved, thanks Tony.
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