Well, I finally finished replacing my X-axis screw with a ballscrew. I actually used one that I bought and had machined by John Stevenson to fit an X1 mill conversion I was doing, but them I bagged a Novamill on fleabay so the X1 got unconverted and sold! The screw is a 5 mm pitch SKF, bought from Arc. The main trick was to remove the nut (usual precautions) and re-fit the other way round so the cylindrical spigot points away from the lathe saddle. (The problem here is that the original Super 7 L/S nut has a spigot 23 mm diameter that fits a bore in the saddle; with a flange to clamp on the saddle face. The SKF nuts have a 24mm spigot.) Then I machined up a simple adapter plate which screws to the face using the original 4 BA bolts, and has 4 M4 tapped holes for the the ballnut. A simple job on the CNC mill. After fitting this everything went back together easily and the performance is much improved, it seems on an initial test to take off just what you tell it to, whereas with my previous plain screw it was very hit or miss. If anyone is interested I'm happy to try to take some photos to show how I did it. I will see how I get on with just converting the X slide, as fitting a ballscrew instead of the main leadscrew looks much harder.
John.