ianRobinson, 
I'm not sure having a second monitor to the right of my main one has been doing my neck any favours.
I’ve removed it for now, and I'm using my iPad Pro as a second screen below my LG 4K monitor for the times I need to have a window on a second screen.
If it's not too glitchy or annoying, I'll stick with this setup for a while.
If it is too glitchy and annoying, I might consider a 5K screen. Possibly an Apple one, or an LG 5K. Which I hear are fine…