I can see that my explanation was rather lacking last night
Before I re-disassembled and re-re-assembled my clutch I could turn the engine over manually with a breaker bar just fine with the spark plug out. The decompression lever should be properly adjusted as per the manual.
Last night I put my clutch back together using sort-of the "penny method" and all was fine, for a short while.
I did not however try to turn the engine over after re-assembling the clutch as I saw no need to at the time. In glorious hindsight I probably should have.
So, I assembled everything needed to try and get the engine started including putting motor oil on the engine, (but not in the cylinder as I oiled that well during assembly).
When I got to the point where I had checked that the decompression lever did what it should, I let the starter try and get the engine running but it got sort-of seized after very little rotation.
I then took out the spark-plug and tried the same, with the same result.
Then I tried rotating the engine manully, and I can rotate it but it takes too much force to do it and it feels like there is a massive friction in the engine (tar instead of motor oil might result in something feeling like this), so I knew something was wrong.
I checked various things, ending up on google and found the thread about some washer on the torque limiter gear that can be misplaced, but could not from that thread understand what washer and where.
A link to a thread with a photo showing it made it clear what I should check, and I intend to check that later today... after having finished some chores regarding gutters filled with leaves, replacing some rubber tiles with concrete ditto aaand some more even less interesting stuff.
Photo from when I disassembled that side of the engine. I can't spot a washer there. Will be interesting to see where it is, if there is one at all...
https://www.dropbox.com/s/0gidj3vfwvutfyy/2016-12-14%2020.40.01.jpg?dl=0I appreciate all the help so far
If you spot anything in my description above that leads to something I should check also, please do let me know,