Actually I worked out the problem from a youtube video, which told me to use:
"magick mogrify... [etc.]"
rather than starting with "mogrify". That worked.
Trouble is, I have another problem now. I've created a new thread about it, here:
https://www.mobileread.com/forums/sh...62#post4263062
I'd certainly be grateful if you could offer any further advice...