As a post script, this photo shows how much thread should poke above the top nut when it is assembled properly. I noticed on the strut where a garage had replaced the spring, that the thread was flush with the top of the nut. I removed the strut, took the spring off, and had a look at the top bearing plate that the nut holds on. I don't know how they managed to do it, but a burr had been thrown up on the inside of the hole in the plate that was blocking the strut tie rod from going all the way through the plate.
I filed off the burr with a small file until the rod went all the way in. Re-assembled the strut, put it back in the car, the nut now seats fully onto the thread as shown.