OK now, I've just done a 'non destructive' system restore using my recovery disk and it seems to have done the trick.
I didn't realise one could recover without losing any data but with Advent, you can, it seems.
OnePlus 5T review: Hands-on
Illustrator Andrés Lozano on his improv line work, brazen use of colours & hand sketching
iPhone X review
Comment envoyer gratuitement des gros fichiers ?