Finally scraped up the cash for proper thrust bearings on the X & Y axes. Got Nachi P4 matched pairs from BearingBoys Ltd


I fitted them in DB or back-to-back style, seemed the best option here - the preload is taken up by the screw nut rather than the bearing retainer.

What a difference, smooth as silk now and no shims needed :)

Will measure backlash again to see what change it has made.