I seem to have fixed this. I found a page (here) with a few extra instructions on suppressing various messages. I haven't implemented them all but it seems the one or two I have added have done the trick! I'd already added one or two of these as per the excellent Retropie docs, but the extra bits here seem to have done the trick. Been mashing about the menu for a couple of hours, and rebooted several times and so far it has not returned.
Thanks for your help. Thrilled to get it working so nice! :-)