Aha, that magnifier is back and, this time, I'm certain that I did absolutely nothing obvious. I noticed this time that, as well as the little magnifying box "on top" of the page, the whole page appeared to be magnified behind the standard page. I went to "Start" to see that "Magnifier" appeared in the left column. Right clicking on "Magnifier" opened a dialogue box and clicking on the offered "Exit" closed the whole magnified page which had appeared behind the main version of the page but left the smaller superimposed magnifying box (which follows the cursor) unaffected.
I have found that, apparently, pressing Ctrl+Esc+R should turn the magnifier on but I can't find anything, apart from the process described in my first paragraph, to turn it off. And even that process doesn't close the little superimposed magnifying box, only the almost hidden full page version.
Help!