Once its assembled again if you hold it by the grey casting in one hand and turn the handle with the other, it does lock up. As far as I can see the bush that sits inside the casting, ( looks a bit like a top hat ), and that the micrometer wheel butts against goes tight and causes the binding. I thought it was the handle working its way along its thread and pushing on the knurled locking disc, which pushed on the micrometer, which pushed on the bush causing it to lock up. So I wound it back a few threads to provide clearance, drilled a hole through both handle and shaft and locked them together with a split pin. No difference, it still locks up! I did think of some sort of thrust bearing somewhere along the handle end of the leadscrew. The trouble is the thread for the handle is too short to allow anything over 4mm to be fitted. Like I said, its been cut off for some reason and is really making things awkward.

It is straight, just the photo makes it look otherwise. The cross slide moves freely and smoothly without the leadscrew.