aaaaaaaand I know see where I was going wrong... I hadn't added a username and password to the calibredb add command
calibredb add /home/craftyclown/Calibre/New/Sonya -r --with-library
http://127.0.0.1:8082/#Sonya --username ???? --password ????
whilst removing --enable-local-write from my service script is the winning combo and seems to allow everything I want to do.