Following on from my earlier reprap ‘wonky circle’ investigation, through a bit of rewiring and rebuild, it seems that the issue is due to a ‘sticky’ x-axis, where the resistance when I push the x carriage back and forth is very variable and quite high in a few places, there doesn’t appear to be anything wrong with the belt tensions. Anyone have any ideas as to the best way to fix this?
https://plus.google.com/118150248616574874140/posts/ax3Uhq4eJKY
Check that your belts are aligned, and that you don’t have some sort of attachment point that is off of where the belt naturally aligns. For example, (if you could imagine) having a belt that traveled perfectly level, from left to right in front of you. But it attached at a point that pulled the belt down slightly off of its horizontal plane. Well, in the middle, that’s not really a problem: the deflection isn’t much. But towards the edges, the belt will get more snug and tougher to push.
Does the resistance issue go away if you detach the belts? What if you just loosen the belt?
The answer will guide your investigation.
Have you tried to take the belt off and see how the carriage moves without it? Could be several things wrong with the mechanics - something stuck in the bearings, rods not parallel or bent. I have gone through the same issues recently and finally found that the belt was rubbing on the sides of the Z carriage. But then, belt issues should show in other prints as well, not only circles.
What are you slicing with? Could be some fancy seam hiding or jitter settings, although I can’t seem to be able to make these work in my Kisslicer and my circles always start at the same point 
Thanks for all the hints - removing the bearing from the one end (not the x-stop end) allows the carriage to glide. I’ve loosely put it back, and things seem to be much better - it might have been a misaligned belt or a small piece of swarf in there… problem fixed, for now. many thanks!