This episode is set in the 1960's. The pirate station "Radio Catherine " is loosely based on the pirate station "Radio Caroline ". Created by Ronin O'Reilly, she was the first pirate station to broadcast off shore.
The station's were on Medium Wave (AM in the U.S.) and at night could be heard in other countries across Europe. AM uses the Ionosphere to bounce off of so the signal can travel farther. This was the reason why listeners were tuned in at night and listen until sunrise.
Many pirate stations were on land and were easy to gain access and be shut down. Radio Caroline's owner Ronin O'Reilly got the idea to put the station on a ship and moving it into international water thus keeping it from the law.
The character Nigel Greybone could have been modeled after Wedgewood Ben, the real life leader of the GPO who tried to shut down the pirates.
Billy King could be based on real pirate Emporor Rosco. Patt Garrett gets his name from the Lawman who shot Billy The Kid.