~~Just when I thought my issues were fixed....!!
Trying to set up version 2 of this Raspberry Pi - everything is the same as the one discussed above, image from v1 copied and then written to a second SD card and inserted into a second Pi 3B+
As I start it up I'm getting the usual "no gamepad detected" which is fine, I'll live with it, but on v2 only, when I try to map a pad (in this case a second Mega Drive replica, and a brand new out of the box second Mega Drive replica) I cannot map any buttons other than Up, Down, Left, Right, Start and Select.
When I get to button A, I have no response whatever. I thought it might be a faulty controller, but as I've said, also tried it with a one I know works on the original system and get the same issue.
Try rewriting the SD card? Anyway to skip button mapping completely? Something else to try?~~
Ignore. Got it working by changing the input mode of the pad from X input to D input