Thanks, Mark. In the elegance stakes, that solution scores zero, I'm afraid! Fortunately, by updating MP Navigator to v4.0.5 and reinstalling the IJ Network Tool in its latest version, all problems with my MG 6150 have been overcome. Took me a couple of hours to sort that out and I would have hoped for an auto-update from Canon, to be honest, but the job's done, thankfully. Hopefully this might help other sufferers.
People, read this entire thread and you will realize that Canon is not going to update MP Navigator for High Sierra. I have called twice and talked to customer service as well as tech support. I will not downgrade my OS for the sake of Canon and insted went out and bought a new Epson printer that does everything (and More) that my Canon MX880 series printer did. Uses less ink, very quiet, fax, scan and print are excellent. Best of all they update their firmware and Scanning App almost as soon as Apple updates High Sierra.
If you have an MX870 there is a better solution. Read my and other previous posts for how to find MP Navigator 3.1.5 that works just the way your old one worked and it is far more covenient than Image Capture
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.