Clive, I don't think this will be the case. More likely the frame geometry.
Is it out by 0.5mm over 200mm ? if so the steps per might be wrong
So I repented my error and got the Z axis within 0.02mm left to right. Just don't know how to correct the 1mm drop over 400m on the Y axis.
Yes you are correct Rob I was reading it as it was out from 400mm to 399mm and not the drop in Z