I sometimes feel... say, silly, dummy...
I spend a couple of hours tracking all the connections, earth and positive, to find that everything was in a rather good nick.
Then I came back to my first check, the fuses. They looked as if they were new, but I took a multimeter to check them correctly.
It happened that the main fuse was not working. It was not blown at the middle, as I would have expected, but broken at an end, and that was not visible.
I changed it, and the bike fired instantly as usual.
The lesson is... check, and double check!
Thanks again!