I seem to actually be getting somewhere after last weekend.

I have mounted the Y axis on the X-axis rails, and built a Z-axis minus a motor mount which will have to be bodged somehow.
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The z axis ballscrews are binding slightly, I think because the plastic inserts (where the balls are returned) are getting squashed by the linear bearings when I do up the bolts on the ballscrew mount. It is fine when they are loose, so for the moment I might get away with some shim under the mounting block and moving the whole ballscrew out a touch.

I am getting to the point where I want to get the thing working, at least so I can use it to make more accurate replacement parts for itself...

Gary is apparently sending me some replacement balls, free of charge... thankyou!