Thanks for your advice, folks.
I was very careful with the job: cleaned off the plates, got rid of all the dust, cleaned everything one last time with thinners to get rid of crap, centralisd the plate, tightened all bolts progressively, tightened to the recommended torque etc etc.
The clutch is a Valeo.
I adjusted the cable again today, and it seems to be better. I'll check out the engine and gearbox mountings and monitor the situation in the coming days.

For next time (ie in 2018, or so) what clutch would you recommend as the better alternative to a Valeo?

Centralising the plate ceases to be relevant the minute the engine & gearbox are mated. If it fits, that's all you need to worry about, since as soon as you start the engine and press the clutch pedal it'll do any "fine-tuning" of the centralising that's needed.

Juddering can also be caused by a warped flywheel, if replacement of a worn out clutch has been delayed and the car driven with the clutch slipping for any length of time.

Also, if the oil return scroll on the gearbox input shaft is badly worn, contamination of the driven plate with gearbox oil is possible, with the same effect...

ken.