Thanks for your reply - and sorry I'm so late getting back here.
No good news, though !!
I still haven't managed to kill the monster
[Admittedly, I haven't had much time to try since I last posted.]
Anyway if I can't
kill it, I'll go back to a "pre" big ship save and just finish all the other quests that I can.
But that would be a disappointing finale after I was enjoying the game so much ....