Just a quick update.
Have mostly finished the lathe work on the ballnut carrier, just needs to spun in the chuck, and the top flange machined down to size, and some bolt holes drilled.
I decided to go for a SKF nut for securing the bearing in the end.
For £7, it saves me a lot of time/effort making my own.

Next part of the puzzle is finding a suitable pulley to machine, and making up a base plate to mount everything onto the mill.