Our HP Z5200 ...

... is now successfully repaired, which is something of a relief.
Installation of a new starwheel lift motor and the associated cams did the trick.  It proved a more straight-forward repair than I feared, and it was certainly much quicker and easier compared to the problems we had last spring with the carriage and service station.
The backlog of large format work has now been cleared, and I'm hoping we don't get any more problems with this printer for a while !