BLTouch doing poor job of levelling
Found the cause of the gaps. it was one or both of the following:: he x-carraige belt was rather loose; I think I'd overtightened the x-carriage assembly with the print head on it and the motion had a couple of places where it was a tad tight. Adjusted both these things and all the problems went away. What's especially interesting is that I'm now getting better quality prints than I've ever had. It looks like it is worth checking the tightness of everything on a new Ender 3 Pro to ensure it is all as it should be!