I'm back.
Several days ago, as I said, seeking advice I looked on this site and quickly found a conversation between two blokes on exactly my problem - caused , one of them claimed, by "hamfisted passengers getting in and out" - slamming the canopy back too violently, presumably. But I've failed to find it again despite putting various phrases in the search box. I thought one of the people was you, Nev, but it seems you are not on intimate terms with the Sierra.
The right-side pin co-operated so I was able to raise the edge of the canopy and replace the sheared pop-rivets of the travel stop bracket on that side, so the travel is now controlled. I'll try penetrating oil on the left handle, think about a modified G-clamp and enquire of Tecnam's U.S. branch about a replacement handle and pin, or if all else fails a latch assembly.
For those of you helpers who have been guessing about something you've not seen, I've attached pictures from the maintenance manual. The steel pin is solid, not a roll pin. Although only 2-3mm diameter and 13mm long, it has a v-shaped longitudinal groove cut in the central part, swelling it slightly and whose sharp edges presumably help its grip. It has a name but I can't think of it...
Thanks to all. It's good to keep the little grey cells active.
Mike
Sierra canopy latch 1.pdf
Sierra Canopy latch 2.pdf