Worked great - many thanks for your help.
BBEdit is superb for coding, but I also like to use it for writing content for Affinity Publisher. Affinity is very nice for formatting, a crime against humanity as a word processor. As are most DTP applications.
So, I use a BBEdit notebook. That aggregates all the text and sidebars in one file which I can copy-paste into Affinity.
I use double-returns to delimit paragraphs, so to paste into Affinity I use Remove blank lines/select all/copy/undo before pasting as plain text into Affinity. I could use a word processor, but that has the complication of pasting new styles into Affinity. Of course, I could paste as plain text, but why not just use plain text to begin with?
BBEdit is a great tool.