@dankcushions You absolute legend! That has resolved my problem. Many, many thanks!

@mitu Thanks!!! I will try this :P

i can't really diagnose without all relevant cfg files (both /all/retroarch.cfg and /whichever-mame/retroarch.cfg) in full and all the rest of the questions asked in https://retropie.org.uk/forum/topic/3/read-this-first. also ideally a screenshot of the issue (can do this in retroarch quickmenu)

if you're getting spam warnings try uploading them to pastebin or wherever

@graf remove /opt/retropie/configs/mame-mame4all/cfg/default.cfg

if you removed the mame4all.cfg you should re-install the emulator to get the default config back