@Floob
Thanks Floob, I thought I was running version 3.8, but can't see it in any of the menu's / splash screen - where would I find it?
I ran the sudo find / -name "default.cfg" command, and there is one in home/pi/RetroPie/roms/config/default.cfg
I also tried the nano /opt/retropie/configs/mame-libretro/retroarch.cfg and tried to put a select button map in there. Whatever I've done (blindly) seems to have helped and I could restart and use select the libretro mame config menu, but not in advmame.
However, in my earlier attempts to update packages to try to fix the button mapping I've created two new problems -
(update - fixed, and no longer an issue, I removed the static IP allocation and it works again!) my WiFi adapter doesn't seem to work anymore (edit: it see's the SSID, but doesn't connect - I'm checking my router config / static IP allocation in case the problem is in there...)
the RetroPie menu in ES looks very different, and
in the libretro Mame config interface (pressing Tab when game is running), half way through setting the buttons, it starts to takes ESCAPE to be a button choice, rather than to clear the existing button mapping (by the way your
was one of the most helpful when I first set this up with a little usb gamepad to test the concept before i built my cabinet - it's brilliant, thank you).