After getting no response and having read the forum on similar issues, I just went back to CONTENT / PAGES.
I don't know enough about MySQL to play with it, so this was a new attempt at fixing this.
I clicked on the link to REORDER PAGES [which I'd never looked at before] and then I had a new list view of all the pages.
I thought I might be able to type in new page numbers, but I couldn't.
What I could do though was grab a page or group and drag them up or down.
I did this a few times, 'saving' after each move, in an effort to force the crazy pages closer to the bottom of the list [for no good reason except it sounded good at the time

]
I did this a few times, then viewed CONTENT / PAGES again - and it was a miracle! those crazy pages had their invisible strings tweaked and straightened out, and now they are back to normal. Phew!
Cheers
WebGirl