After an hour or so of hair-pulling, it's not enough to add the new boards manager URL " https://raw.githubusercontent.com/espressif/arduino-esp32/gh-pages/package_esp32_index.json " - you also have to remove other esp32 URLs no longer in use, and put the above at the start of the list.

I removed thes and it finally gave me the new 2.0.4 version to update to:-

https://raw.githubusercontent.com/espressif/arduino-esp32/gh-pages/package_esp32_index.json
https://raw.githubusercontent.com/espressif/arduino-esp32/master/package.json