I did try the bootsect.exe without success. Actually the boot folder on my dvd didn't have it, but I have recently burned a Win7Pro 64bit ISO and used the bootsect.exe in that disc.
Strangely, my original Win7 Ultimate 64bit disc (as photographed), and the newly burned Win7Pro 64bit disc both produce the same error when I boot off the disc. They both say "This version of Windows does not match the one on the sytem" or something similar. It's a VBOKOnly dialog so I can't access startup repair. Rather strange as during this long process I was previosly able to boot off my Win7 Ultimate disc and use the system repair tools there.
Anyway, I don't expect that will be a problem if I format the drive and start fresh. That appears to be my last resort now and I'm going to do that probably this weekend. The latest Win7 Pro 64bit ISO on the HEIDOC.net site (called "Media refresh") is what I'll try to use to format and then re-install, using the original product key on my pink and white sticker under my battery. As you say, once that's installed I can try restoring a system image. At least then I'll have a working computer!!
Will let you know how that goes. Thank you so much for your long-standing patience and support.
Perhaps at this point I can ask about recommendations of installation with dual-boot? I think it would be great to have a linux (ubuntu) OS on the drive to future-proof against similar Microsoft self-destruction..."