Wait a minute! Just read this - 'After installing RetroPie on an SD card, and before booting it up, make sure you edit your /boot/config.txt and add or uncomment ( remove the # before ) hdmi_force_hotplug=1.
Since the display is powered from the Pi it wont be properly detected at startup, so this is required to make it work’
I’ll give it a go tomorrow !
Nat