@fortMaximus Thanks. The 2 images are me trying to get the Thumbnail image + caption to work....the 1st is as per Quincy's code but behaves differently, and the 2nd is me adding styles to the thumbnail and caption text to try to fix it.
So I've learnt that with CodePen + Bootstrap CSS library added, not all the Bootstrap classes work....so I need to stick with core HTML + CSS and code from scratch.
@fortMaximus Cracked it! CodePen adds the latest alpha of Bootstrap 4.....but when I delete that and use version 3, everything works.
So I now have a stable dev platform to work with. Yeah!
https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css <== Worked (copied from Quincy's settings)
https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.0.0-alpha.6/css/bootstrap.min.css <== Didn't work (from "Quick -add" of Bootstrap)
I can now delete everything and start from scratch.