Croydon is a town in Surrey which is the home of "Martin Smith". When Arthur Dent, upon meeting Zaphod Beeblebrox, unexpectedly tells Ford Prefect that they had met previously, Ford insists that Zaphod is an alien, "not bloody Martin Smith from Croydon". It turns out, however, that Zaphod has previously visited the Earth and indeed met Arthur (very briefly) under the false name of Phil at a party in Islington on the very same night Zaphod picked up Trillian.
Behind the scenes[]
Martin Smith was in fact one of Douglas Adams' co-writers during his Footlights years at Cambridge - and, yes, he did come from Croydon...