So its seems like this is some kind of clash between the Intel GPU and the Nvidia card. Tahacannn on the Unreal Questions page found a work around.
Basically start the editor with the -dx12 argument and get the project to the point where ray tracing is working, or just open the project if you are already at that stage.
Minimise the window (this is important) then open the device manager and disable the Intel display adaptor. Then you seem to be able to use the editor as expected.
For whatever reason, this doesn’t work if you disable the Intel GPU before opening the editor…