The new film from director Richard Curtis, creator of Four Weddings and a Funeral and Notting Hill, looks back nostalgically to the days when a handful of pirate radio ships with vast radio transmitters bobbed about in the North Sea broadcasting to the UK. The BBC monopoly of the time meant that independent radio was illegal on UK soil; commercial broadcasts had to be illicitly beamed in from continental Europe, or from shipboard transmitters anchored around the British coast.

Set in pop’s golden age of 1966-1967, The Boat That Rocked is about the fictional Radio Rock, a floating radio station whose roster of DJs live in clannish male conditions aboard a decrepit trawler. It’s modelled on real-life pirates such as Radio Caroline and Radio London, which from their moorings several miles off the Essex coast beamed a constant diet of pop songs to as many as 25m people, half the UK population.

The pirates, launched by entrepreneurs, funded by advertising and staffed by irreverent young DJs, filled a vacuum left by the fusty BBC, whose meagre ration of one hour’s pop broadcasting per week dismally failed to meet public appetite for the cascade of hits issuing from the Beatles, the Beach Boys, the Rolling Stones, et al. It was, said Curtis at a screening this month, “the worst mismatch of supply and demand in history”.

His comedy portrays life on a pirate boat as a boyish blast. Dolly birds are shipped in, dope is smoked, drinking games are played, banter is swapped, groovy tunes are played – while fuming little Hitlers in Whitehall struggle to wrest back control of the airwaves.

Tony Blackburn joined Radio Caroline in 1964, several months after its launch, and went on to become one of the best-known figures in British broadcasting. He remembers his pirate days fondly. “Just terrific memories,” he says. “What an adventure.”

Yet it was not the bacchanal depicted in the film. “There was nothing like that,” Blackburn says, referring to on-board parties and shipments of girls. “It was run very professionally.”

“As any old naval person will tell you, when you get women on board there’s trouble,” adds Roger Day, who swapped his day job as an accountant to become a DJ on the pirate Radio England in 1966, before moving to Radio Caroline. It was a brash, testosterone-heavy environment, but Day and Blackburn insist that clashing egos were rare.

“People who didn’t get on with other people didn’t last very long,” Day says. “They either conformed, or they were got rid of. If you’re locked on a boat for two weeks, the last thing you want to deal with is an awkward devil who’s trying to rock the boat, if you’ll pardon the expression.”

The fortnight stint at sea began with a 90-minute trip in a small boat from the Essex port, Harwich, to one of the pirate ships. The levels of comfort varied. Radio London, which Blackburn joined in 1966, was owned by Texan businessmen and housed in a large US minesweeper where the DJs had their own cabins. Radio Caroline, founded by an Irish impresario, was more cramped. Cabins were shared – Dave Lee Travis, aka the “Hairy Cornflake”, was one of Blackburn’s roommates – and a large mast caused the boat to sway alarmingly in bad weather. It was not a job for landlubbers. “If you came on and you were seasick, that was no good,” says Blackburn, whose upbringing on the Dorset coast proved useful. “I was quite an old salt before I got there.”

Caroline’s boat, called Mi Amigo, flew under a Panamanian flag and was crewed by Dutch sailors. She evaded prosecution by anchoring herself outside UK territorial waters. Maritime hazards were common. Blackburn was on board when a storm forced Mi Amigo to run aground at night off the east coast resort of Frinton. As waves crashed over the deck the captain shone a searchlight to reveal people walking on the seafront 400 yards away. “He shouted out, ‘Mayday!’ and I remember saying to him, ‘I think it’s too late for that’.”

The pirates’ heyday ended in 1967 when the Labour government passed legislation banning firms from advertising on them. In 1968, Mi Amigo was towed away to Holland by Radio Caroline’s creditors. Roger Day was on board. “It was the saddest day of my life. I’d found the perfect job and there was this bloody government taking it away from me. I’ve never been able to forgive them for it.” The station came back to life afterwards (indeed, it still broadcasts on the internet), but it was a shadow of its former self.

“I remember the first day I went out there, on July 25, 1964, and I saw this little ship and I thought this is going to alter the whole of broadcasting in this country,” recalls Blackburn. He was right: Radio Caroline and its fellow pirates revolutionised British radio. Jingles, competitions and top 40 countdowns were part of a culture imported, like many of the pirate DJs, from the US. The BBC set up its pop channel Radio One in 1967 to challenge the pirate ships, and recruited many of their stars, including Blackburn, Johnnie Walker and Kenny Everett.

Above all, the pirate boats promoted the DJ as personality. That personality could be laconic, as in John Peel’s case, or manically chummy, as with Dave Lee Travis, but either way it marked a break from the BBC’s traditional patrician hauteur. The likes of Blackburn – a 21-year-old would-be singer with a business diploma when he joined Radio Caroline – and the former accountant Day helped bring a demotic voice to British broadcasting.

“You’d go in a pub and everyone would crowd around you,” Day remembers. “As an accountant you’re not used to that sort of attention. I guess it was like footballers are now. We were pop stars.”

‘The Boat That Rocked’ is released in the UK on April 1 www.theboatthatrocked.co.uk

Roger Day will broadcast a pirate radio tribute from a boat in the North Sea on BBC Essex on April 10-12