I did follow all those steps and adding the repo must have worked or apt search wouldn’t have found the packages.
I’ve just tried following the same instructions on a clean Debian VM and it worked. Consequently, it’s clear that the difference relates to the way the two Debian installations are configured. Both are fresh installs but the one that worked was a full desktop installation whereas the one that didn’t was a cut-down text only installation.
I’m going to start from scratch again only doing a full desktop installation and see whether that works.