After struggling with the Z movement being unpredictable, I decided to clean the whole thing up and give it a good working over.
Then I found the culprit.
I among the sawdust at the base of the router cradle were a couple of little ballbearings.
Heavens only knows how many of them I have already vacuumed up but the bearing(s) is/are definitely shot.
Thank goodness I have a couple of spare ones.
Time to take the whole thing apart and fit new bearings