The scene in which Young Carl sulks about Marianne was originally supposed to be about eight seconds long and contain nothing but Tom Sturridge sitting on a bench and looking sad. Will Adamsdale and Ike Hamilton happened to be there, so Richard Curtis told them to have a go at what they would have done if their friend was in the same situation. He put on 'So Long, Marianne' and they came up with everything that remains in the scene.
The ship used for filming was the MV Timor Challenger, and much filming, both day and night scenes, was done in Portland Harbour (venue for the 2012 Olympics sailing) near Weymouth. The red paint scheme was based on that of the MV Ross Revenge, which was the Radio Caroline pirate ship from 1983-91, and is nowadays maintained as a museum by volunteer enthusiasts, who assisted with several aspects of the film, including loaning many of the broadcasting and ship props.
Although purely fictional, the film does reference many of the actual events surrounding pirate radio. Namely, like the pirate radio ship, the Ross Revenge', the movie's ship is red with the station's name in white paint on the deck. Radio Caroline's ship, the Mi Amigo sank, complete with fleeing DJs unable to save the ship's vast record collection and as in the film's finale, its extremely high mast rose out of the sea where it stayed for many months. Also a DJ did marry on a pirate radio ship as Simon does in the film. Additionally the DJs are seemingly composites of popular radio personalities of the time such as Tony Blackburn (also unlucky in love in his early days ala Simon Swafford, when his rocky marriage become apparent in his radio show) and the wacky Kenny Everett has traits similar to Angus Nutsford. Dave Lee Travis, Whispering Bob Harris, American DJ Emperor Rosko and Johnnie Walker also are seemingly portrayed in the film.
This film is very loosely based on Radio Caroline, a popular pirate radio ship with a similar history and style. It was director Richard Curtis's intention to weave a fictional story around the many pirate stations of that era, rather than base the story on a factual story.
Though the film is set in 1966, much of the music featured is anachronistic. For example, Bob carries a copy of The Incredible String Band's 'The 5000 Spirits or the Layers of the Onion', which was neither recorded nor released until 1967, and The Who's "Won't Get Fooled Again", used to score a climactic scene, was not released for another 5 years.
Hans Zimmer wrote the film's score, which consists of only one six-minute cue during the finale, without being credited as composer. He did this as a favor to Working Title, with whom he had worked during the beginning of his career in the 1980s. The score track is listed in the closing credits among the numerous songs used as "Sink or Swim" and Zimmer is listed, in the fine print, as the writer of this particular "song" along with Lorne Balfe.
While the historical character who was responsible for shutting down pirate radio was Tony Benn (the Postmaster General at the time), Alistair Dormandy, Kenneth Branagh's character, appears to be a synthesis of uptight and mean spirited and outdated 1950s Tory patriarchs.