Posting this in case it helps somebody else, as in my case the fix was a relatively simple job.
One of my seat base actuators suddenly developed a very notchy movement accompanied by a loud click. Peeling back the seat cover showed that the LHS paddle was out of position, due to the pivot rod being disengaged.

Closer inspection revealed that the Allen grub screw that secures the retaining ring to the low end of the pivot rod had come loose, the retaining ring had subsequently dropped off, and the rod had come out of the hole.