@J-M-L , @anon57585045 ,
Ruined it? You have all completely destroyed it! 
Seriously, all the comments are very helpful and appreciated. I have created a new version here: Flashing multiple LEDs at the same time, which I intend to keep 'clean'. I've invited @J-M-L and @Paulpaulson to contribute their code so there are 3 versions for people to study.
I am grateful to all the other contributors to this topic, but, in the spirit of only presenting different ideas for non-blocking blink code, I have not included them in the new version. If any of you have a version that's substantially different and contains something new then I'm happy to consider it. Please post here.
I thank you all.