Downloading Originals to this Mac selected - but doesn't seem to download

Thanks, Léonie. Yes, that did force downloads - but it was going to be a lot of fiddling. I played around and found an earlier discussion in the "Apple Support Community" that dealt with force downloading onto an external drive by starting a new library (from '22 with input from you). So I set up a new library, but in the "Pictures" folder rather than on an external drive. I made it the system photo library and in setting it up ticked "Download Originals to this Mac". It worked - quickly sync'ing with iCloud and then slowly downloading all the photos (it's a big library and my connection isn't that fast). I don't know why it wouldn't work with the original (problem) library. Maybe selecting download at the time of setting as the system library would have done the trick or maybe there was a problem because it was from a backup from the old computer and had been set up with "Optimise Mac Storage" selected. Regardless, I managed to download and so all is good.