In the mean time have been able to get Debian Buster running. Installed mopidy through apt-get. Now if I want to install mopidy-mpd, it reports mopidy needs version 3 and it appears version 2.2.2 was installed.
I have looked around, checked the FAQ above, but how can I get version 3 on my system? Surprised it’s not on the repo.
Edit: deleted the apt-get repo version of mopidy, removed all and then installed the pypi version. It installs all the right dependencies and just seems to run fine 
So the debian repo might need fixing, but the main stumble block seems a too old python3-pykka on the repo.