Bit of a set back today. I noticed that the Z screw was operating smoothly in one direction but lightly jammed and clicked when the direction was reversed. I've taken it off of the machine and now believing there must be a ball trapped between two circuits, however I've now re-packed it twice and the exact same problem persists. I think there is probably a small error where returns are machined leaving a tiny gap that allows one outer circuit and the middle circuit balls to touch. The plastic returns didn't look to clever to me.

Anyway I've contacted Fred from BST to see what he suggests.