Circles aren't circles on the CNC....

[JAZZCNC]

I had an thought it could be this although x and y steppers are identical but x seems to be the one losing itself ever so slightly. Out of interest how do you change the steps? is it the motor tuning in mach3?

Yes, it's in Motor tuning. Make sure you click the Axis selection on right before making any changes and save when finished.

However, if the settings are identical it could be the Step pulse rising edge for the X-axis drive is on the wrong side. If the Step pulse is firing wrong you will lose 1 step with every direction change.
The way to test for this is to write some G-code which makes lots of A to B moves and back to A. 200 or 300 short moves will do it IE:
G0 x10
x2
x10
etc, etc just copy paste 2 lines above finishing with.
x0
m30

Then Zero Axis and make a Mark, set the code running and when it's finished it should return back to Zero and the mark. If it doesn't return to the mark your Step pulse needs changing.
To do this go into Motor Outputs and change the Active Hi/Lo for the Step pulse.
Only do this if it doesn't return to mark.

Check all axis this way if unsure.