Right. I've done the mesh levelling properly.
Did a quick first layer test (that I sliced), I did it twice, once from Marlin and once from TFT (wanting to put my mind at rest that TFT was doing mesh levelling)
Both came out exactly the same, both looked perfect while on the bed
But both of them, when I removed the squares from the bed, the left hand edge on the largest two squares separated
They were printed with bed at 60 and nozzle at 200
I tried printing the level test I printed when I first built my printer (downloaded gcode, no slicing by me) , this printed at 195 and the bed was 45, this printed 100% fine on both Marlin and TFT and when I removed it, zero separation
So I'm happy everything is working, and I just need to learn how to use Cura properly
As a final test, I printed a dog that I didn't have to slice.
Very happy with the result. Still need to calibrate extruder, that's my next job
Many thanks for all your help, I'm sure something else will screw up pretty soon