I've now got it.
Thanks TIG (and additional gratitude/ kudos for you ruby plugins and contributions). Thanks also J Bacus, Gaieus.
For some reason, it's been a bit af a learning curve - I'm probably still dumb, but less ignorant thanks to you all and the forum!
TIG, the process you describe was pretty much the way I went about my first attempt. The breakthrough came when I read John Bacus' comment aand realised that by zooming in close I could improve the image quality, and that the new "Add more imagery" system allows me to quilt together a terrain. I work mostly with golf holes and imagery needs to be long and thin, so with a single image in SU7, the orientation was important. In SU8 this is no longer true.
BTW Gaieus, thanks for your suggestion for SU7, I had tried it but had more reliable results for the GTS method. But hopefully, its one for the history books.