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.
Luckily this happened on the low end of the rod and the ring was resting next to it. If it happened on the upper end the ring might have fallen into the actuator mechanism and been tricky to recover.
Repair was a simple case of reinserting the rod, feeding the ring back on and tightening the grub screw. I say 'simple', but space and visibility are limited down there, so it's a little bit fiddly.




Anyway, hope someone finds that useful in case they experience the same symptoms. :)