Hi @davedoode,
You’ve got the right idea, the trouble is you’re running in to your browser’s same-origin policy, which prevents loading generic data files (like json) from the file system. The three.js project has nice page covering this concept and some ways to work around it (see https://github.com/mrdoob/three.js/wiki/How-to-run-things-locally).
Basically you can open the html file using a webserver (not as involved as it may sound, you can do this using a single Python command) or change your browsers security settings.
Hope that helps!
-Jon