Thank you.
I got it to work. I even tried the Segoe UI Emoji font and found that I could insert a character, in fact I used 1f335, the cactus. Not only that, when I produced a PDF document (in fact, a 'for web' PDF) and then copied from the PDF document to WordPad, the character then pasted across. This was as a character, not as a glyph, which I proved by then copying it from WordPad and pasting it into the Edit Search facility of the High-Logic FontCreator program and got the 1f335 as the result.
I repeated that with a character from an experimental font of my own and got back a code point of f9008, which is correct.
Gold star to the Affinity team for producing Affinity Publisher using all of the Universal Character Set and having the character codes go through to a PDF document and having them able to be copied out. This is a major step forward.
William