The immediate Christmas jollity now out of the way I've been able to return
to the question of Win 7 and Win 10 in the same desktop PC.

With each OS on a separate SSD, pressing f8 during bootup brings up a list
of installed drives and other gubbinses, ending in Windows Boot Manager,
which when selected lists both 7 and 10. So so far, so good.

Selecting 7 takes me straight to a Checkdisk check of drive C, the Win 7
SSD. This finds no problems and 7 then loads and runs.

Selecting 10 brings up a message to the effect that Windows has detected
unauthorised hardware or firmware changes (I quote from memory but that's
not far off) and I'm asked to insert a Win10 repair disk. I haven't tried
this yet, but it's next on the agenda.

Wish me luck...