AHA 
It was cloudflare. I had no problems with the v0.1 site using Online mode, but with Synced mode it has apparently become necessary to bypass Cloudflare auth for the service worker etc. as per your guide Cloudflare and Portainer, following this part closely:
Now it all seems to work.
@zef thanks for your patience, hopefully this will help someone else in the future. Why are the bypasses required?
Nick