Thanks everyone for suggestions. Switch bounce was one of the first things I thought of, so changed the mouse - for a corded one, as it happens. No change. Tried changing from hibernate to sleep: no change. And yes, SD, I was wondering about something like hyberfil.sys and ssd corruption. Soooo,
AFter much teeth-sucking I've decided to go along the route SD suggested in post 2. I have a spare ssd, have downloaded a three-machine copy of Windows/Office from a site promising that they are a M$ partner, and will take out the old disk & replace it with a clean one; install all the stuff again...
and if that doesn't do the trick i shall have to leave it on the back seat of an unlocked car.
One will report in due course.