I have got it working now with the Adafruit library and have posted details here :-
https://forum.arduino.cc/index.php?topic=624003.0
SFEMP3Shield library
There are assorted versions.
They don't seem to be supported for ESP32 so it needs assorted code changes to get it to work (which I haven't got to the bottom of)
Configuration of the pins is necessary in the library.
They use SdFat..h
I got confused by all the old information as SdFat looked to have superceded SD.h for the Arduino, which originally had 8.3 file length naming restrictions
but SD.h in ESP32 library is fine.