OK, figured out my issue.
I'm running my web server on raspberrypi. This does a reverse proxy to my Windows10 IMA PC (used solely for IMA and also Backblaze backups)
I had not updated the imatchviewer directory on my server since goodness knows when (might be 2015, as this is the date of the original config.json

) . When I replaced the imatchviewer files on the server (and made the appropriate changes to the config.json files) I get the new heading with the slide controls in the web browser.
All resolved!
Aubrey