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Behold the new Picade from Pimoroni (for the record, I am pronouncing that Pie-cade from Pim-moroni, but you can please yourselves).
RetroPie, as would I, so I downloaded that and set it writing to the SD card whilst we finished the build. You also need a power supply, and one of the official 2.5A power supplies is recommended.
hdmi_force_hotplug=1' in the config file fixed it. Right, it's booted, time to run the script to install the Pimoroni settings.