@dc42 I'm using the Duet Smart Effector. Version 1.3 as marked on the PCB and programmed with the default sensitivity.
I took some photos, but not sure what might be useful; let me know if there are any specifics that you would be interested in seeing.
To be fair, now that I have managed to update the firmware and am running a bed mesh with more points, the error has become within 2mm on the one corner. It could be that I am chasing precision that is not achievable. It is significantly better than previous!