Thanks for the replies so far!
Unfortunately I don't own a TTL/USB converter, although it is on my shopping list. Is there any way I can modify an Uno board to accomplish the same thing? Like pulling the MCU out so the 8U2 HAS to talk to whatever I've wired up to the Tx/Tx pin headers?
I did remember about the blanking pin :-)) I've got the display to light all segments using synchronous mode so I'm fairly confident the hardware actually works, and the fault lies with me.