As Richard Curtis explained on the Dave Cash BBC Radio Kent show of 28 March 2009, this movie is not intended to be a depiction of the real story of offshore broadcasting to the United Kingdom as it took place in 1966. Curtis told the audience that someone should make such a movie but that his movie is for entertainment purposes only.
Alistair Dormandy, Kenneth Branagh's character, is a parody of Tony Benn, United Kingdom's Postmaster General in those times.
Filming took place on the former Dutch hospital ship Timor Challenger, previously De Hoop, moored in Portland Harbour, Dorset; the "North Sea" scenes were shot off Portland Bill, while boat interior shots were filmed inside a warehouse in Osprey Quay on the Isle of Portland and at Shepperton Studios.
It's the fourth time that Emma Thompson and Kenneth Branagh work together after Dead Again (1991), Peter's Friends (1992) and Much Ado About Nothing (1993). However, it's the first time where both don't appear together in the same scene.
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.
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.
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.
Dormandy's secretary was originally known as Miss Clitt in early versions but is now only known as Miss C, presumably due to much being made of having two character names doubling as innuendo.
Rhys Ifans' character, Gavin Kavanagh, is probably based on 'Admiral' Robbie Dale, who was a long-term pirate DJ.
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.
Scenes were filmed with James Corden as a rival DJ sabotaged by the Radio Rock crew. These were cut from the film but are available as deleted scenes included on most home video releases.
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.
The scene with Midnight Mark and all his naked groupies (cut from US release) is an homage to the UK album cover of Jimi Hendrix's album 'Electric Ladyland', even down to the placement of the girls.
The album Bob grabs at the end is the 1967 record, The 5000 Spirits Or The Layers Of The Onion by Incredible String Band.
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.