But still, it's all simple really.
We just need there to be an alien civilisation about 25 light years out, who'd developed the noise/signal sorting tech by late 1978, so they could pick up Doctor Who from the start as the BBC signals reached them.
Being people... ah, aliens of taste, they would instantly realise it was so good that it should be re-broadcast on their own TV stations. Which means we can pick up their retransmissions of Marco Polo early next year, if SETI just develops the necessary noise-signal tech in time!
Mind you, it'll be really irritating that, after Marco Polo, we'll only get six more missing episodes off the alien TXes until October 2015. ;-)