Ok, sorted it i think

Couldn't rest without a fix so a couple of hours fiddling and a few thin shims later we have smooth motion from top to bottom, its pretty easy to reverse-drive it by moving the quill lever

The bracket that holds the nut was slightly twisted and the nut hole was too tight, fixing both of those cured the problem.

Now i need to fix up some limit switches and get the motor hooked up.

Click image for larger version. 

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