Thanks, I'll explore the EEPROM suggestion (also made by @DVDdoug), although I've not so far used EEPROM in any project.
I was again frustrated by being unable to copy/paste from the serial monitor, so here's a screenshot of the run taken a few minutes ago. At a glance identical to the earlier one.
I resolved the (unrelated) puzzle over the inconsistent printing by adding a 500 ms delay before each pair of print lines. Probably could be less, but 100 ms was inadequate.
I saw another poster reporting repetition of 7, 14, 21, which is interesting as mine is 8, 15, 22.
