Right-o! Managed to fix it by working around the "can't add local files" problem:
- Uninstalled third party version of Spotify:
sudo eopkg rm spotify
- Installed snap version of Spotify:
sudo snap install spotify
- Used suggestion from this comment to copy local files configuration file across to snap config:
cp -f .config/spotify/Users/<username>-user/watch-sources.bnk snap/spotify/current/.config/spotify/Users/<username>-user/
Note: I think this last step only worked because on an earlier version of Spotify I'd been able to add in local files (and hence the watch-sources.bnk
file existed).