In case it's of use to anyone else …
I was trying to insert both the leadscrew and the drive shaft at the same time to save time and avoid disconnecting them from the bearing block. Once I removed the drive shaft circlip it became apparent that it was the drive shaft that wasn't fully entering its clutch so a bit of jiggling and a gentle tap soon fixed that and all is now well. The only thing I'd add is how important it is to slip something through the leadscrew cover to hold it compressed – an absolute bu&&er if you don't.
I'm now moving-on to replacing the tool post (or "tool clamp post"
and see that there are 3 grub screws around it in the compound slide, but I don't understand their purpose. I'll start another thread for that question.