Update, having messed around with all of the PR settings. Making sure Global Clipped Plane setting is ON, all sorts of combinations for the Priority settings, and adjusted the clipping plane values in the project settings - after 3 days +, I’ve given up on the PR!
I then went to Lumen for the reflections and found Marketplace scene had an odd parameter in the water material that was casing the dithering. Normals UV Tiling was at 3000 - I’ve changed it to 1! Once I found this, I was able to get the water looking good without the PlanarRefelction. Perhaps “this is the way”?
The important thing is that this works via nDisplay, unlike the PR!