Thank you all very much. This has been most educational and useful.
I think I may have caused myself some problems, in that it turned out one of my rectangles, had become a curve and had got duplicated several times, without my noticing it. I do not know how, because I never intended any of them being a curve.
Lots more to learn and thank you for letting me be productive in achieving something, since the early days of Corel Draw.
mcl got a reaction from A_B_C in Affinity Designer : Edit corners of multiple selection