Well the mystery has been solved, not something that I thought of previously I have to admit.
I removed the M410 from the Amazon, and found to my dismay that I had put the clutch friction disc in the correct way, the pilot bearing was also smooth and functioning well. This meant that further investigations were required. The hydraulic clutch fork setup in the 1800/M410 bellhousing was also cleared of any wrongdoing.
So I set up a flywheel in a vice, and bolted the clutch assembly to it. Using a bolt and a series of washers, I tightened the bolt to compress the spring diaphragm until the clutch disengaged. Only problem was that one side of the pressure plate was properly disengaged, whilst the other side was still slightly engaged, causing the transfer of unwanted rotational energy to the gearbox. The pressure plate was therefore not co-planar (is that the correct term?) to the flywheel face when supposedly disengaged, bugger. (For anyone wondering, I made sure to compress the fingers equally).
Also, trying for more clutch fork movement (using different diameter master cylinders / slave cylinders) eventually gets you properly stuck as well, since a kind of re-engagement occurs if one over compresses the pressure plate.
Luckily I have several other pressure plates at hand in my workshop (who doesn't….), and on testing those, no problems were identified. so I will reinstall the clutch / transmission next weekend.