Ok, what happened between mpv versions is this. In my emms startup code, I had emms-player-mpv-ipc-method set to nil. This means emms detects the input method.
In the previous mpv version this came back as 'ipc-socket and everything was fine. In the current mpv version it came back as 'file which wasn't fine at all and couldn't work as in emms startup I set command line optons for 'ipc-socket.
However merely changing 'nil' to 'ipc-socket' (for emms-plaer-mpv-ipc-method) didn't work either and resulted in mpv never even starting (from emms).
So I had to create ~/.config/emms/mpv.conf and add the line
input-ipc-server=/home/bnewell/.emacs.d/emms/mpv-ipc.sock
which is specific to my configuration --- the path to the socket is actually what's required here.
So no, this is not an emms problem per se. No bug but mpv now works differently so I had to slightly change my emms setup.
Mahalo to everyone for their advice and ideas.