In 1974, John Lennon was startled as he was watching Top of the Pops. He rang Ringo Starr. “You won’t believe what’s on television,” he reportedly said. “Marc Bolan is playing a song with Adolf Hitler.”

This was Sparks, performing their glorious pop opus This Town Ain’t Big Enough for Both of Us. “It was equidistant between the Beatles on Ed Sullivan and the Daleks on Doctor Who,” says Edgar Wright, the director of Baby Driver and Shaun of the Dead, and now a documentary about the duo, The Sparks Brothers. “Fifteen million people saw it. Think of the next generation of bands watching: the Sex Pistols, Siouxsie and the Banshees, Duran Duran, Joy Division, Squeeze, Vince Clarke. They’re all watching and they’re all thinking the same thing.”

The Bolan in the scenario was Russell Mael, a strutting falsetto singer with luscious locks of flowing curls. Next to him was his elder brother Ron, sporting a toothbrush moustache, sitting motionless behind his keyboard, deadpan expression but rolling his eyes with the air of a man forced on stage to be a mime as a form of punishment.

The performance turned the Los Angeles duo into a sensation in the UK. Hundreds of thousands of singles were sold – just missing out on No 1 – and soon mobs of screaming fans invaded stages at concerts, ripping the clothes from their scrawny bodies. In a more just world, the song that catapulted them there would possess the same ubiquity, and be sung with the same roaring drunken gusto in pubs and karaoke bars across the world as Queen’s Bohemian Rhapsody (which followed a year later), but stratospheric stardom, Live Aid and an Oscar-winning biopic were not in Sparks’ trajectory.

However, after more than 50 years of unpredictable pop that has explored electronic, new wave, art rock, glam rock, neoclassical, opera and just about every other conceivable niche, 2021 may be the year of Sparks. As well as the documentary, they have a film, Annette, which opened this year’s Cannes film festival. The brothers have written the script and music for the experimental musical, directed by Leos Carax (Holy Motors), about an outspoken standup comedian, Henry, (Adam Driver) and his opera singer wife Ann (Marion Cotillard), and how their life is turned upside-down by the birth of their child, Annette, a mysterious girl with a unique gift.

Carax and Sparks connected at Cannes in 2013 but their relationship goes back to when the 14-year-old Carax spotted an LP featuring the brothers tied and gagged on a speedboat. “I stole a copy of Propaganda because I liked the cover,” he says. “They’ve been part of my life ever since. Their tracks are among the most joyous I know.”

“I guess sometimes crime does pay,” laughs Ron, speaking from Cannes after the premiere. “The reaction of films here can go sideways: they really make it known what they think, so to have a five-minute standing ovation was incredible, a really special moment.” Reviews have been varied, with some calling it a masterpiece while others are perplexed. “It’s provocative and completely uncompromising,” says Ron. “It’s the kind of film people will be passionate about one way or another.”

He is clearly elated. “It’s been unbelievable. Extraordinary and dreamlike. Also, we’re such geeks. so having Spike Lee being the head of the jury ... we’re just kids in this weird wonderland.”

“It couldn’t happen to two nicer guys,” says Wright. “I joke with them, saying: what if a time traveller came to 1975 and saw you guys all glum over the Jacques Tati film falling apart and said, ‘Don’t look so downhearted, wait 46 years and another French director is going to make good on this.’” The Tati film – which would have starred Sparks as American television execs trying to modernise a rural TV station in France – is one of two major film projects that fell through in their career. The other was adapting the Japanese manga Mai, the Psychic Girl into a musical with Tim Burton directing, before it collapsed.

So, did they expect more of the same? “In a naive way. we always assume something is going to happen,” says Ron, 75, when I speak to him and his brother prior to Cannes via video call. With his moustache and drawl, Ron is the more comically dry of the pair. “We decided the past was irrelevant. We may be foolish but we thought this would happen.” And now that it has? “We try not to be too self-satisfied but there is some vindication that after all this time it actually happened.”

This is the essence of Wright’s documentary on the eccentric, charming pair: a band who, despite moments of commercial success, always experiment and reinvent even if it results in indifference. “They always seemed ahead of the game,” says Wright. “Oblivious to what else was going on and just forging ahead – like space travellers who have no idea what’s going on back on their home planet.”

Sparks were made from a couple of pop-obsessed boys growing up in 1950s America. In love with AM radio, cinema, surfing and entertaining from an early age, a prepubescent Ron was already dazzling audiences in school talent competitions, wearing a bright pink suit.

They formed early bands – Urban Renewal Project in 1967, followed by Halfnelson in 1968 – and played gigs wherever they could, from dog shows to pizza parlours. They made demos but record labels expressed zero interest. However, Todd Rundgren saw something and offered to produce Halfnelson’s debut. They reissued it under their new name, Sparks, and it picked up a little steam, largely because of the infectious glam-pop single Wonder Girl.

The Electric Prunes’ James Lowe produced their second and was convinced of its magic. “I told my wife, if this album doesn’t make some noise then I’m going into another business,” he says in the film. “I don’t know anything about music if people don’t like this stuff.” He soon started directing television commercials.

Sparks battled on in Los Angeles, surviving on food stamps. They loved the city, and its glamorous Hollywood roots, but the music scene less so. “That Laurel Canyon singer-songwriter vibe was really not appealing,” says Russell, 72. “Like: ooh, it’s got to be honest and in blue jeans and a boring shirt. Whereas putting on a show and being really flamboyant wasn’t considered honest or authentic.”

Instead, they embraced the sonics and aesthetics of the British invasion bands. “I was trying to be Mick Jagger or Roger Daltrey and missed the mark by a few thousand miles,” says Russell. “But something else emerged.” They signed to Island Records for 1974’s Kimono My House – which captured the glam meets art-rock stomp of the day – and broke through.

Sparks released three albums in the next two years, ending on the hard glam of Big Beat. However, by 1977’s soft pop-rock Introducing, they felt lost and threatened as punk exploded. “We were against the wall,” says Ron. “Creatively, but also as far as people finding what we were doing interesting. We needed to reinvent ourselves in a really drastic way.”

Like many, from Eno to Bowie, the pair heard Donna Summer’s I Feel Love and saw the future. “In desperation, we lied to a journalist saying we were working with Giorgio Moroder on our next album,” says Ron. “She was friends with him and so we had to confess but she helped us set it up. It was a desperate situation but – even though some criticised us as being traitors to the rock credos – we knew it was the right thing to do.” The resulting album No 1 in Heaven, made with Moroder in 1979, is some of the most wildly inventive and euphoric pop music ever put to tape.

“All pop music is rearranged Vince Clarke and rearranged Sparks,” declares producer and songwriter Jack Antonoff in the documentary. He is one of many notable names – Beck, Flea, Björk, Tony Visconti – to pop up, and another is Clarke himself. Sparks was the first single he bought and he played it until the groove disintegrated. Clarke’s Erasure also took inspiration from Sparks’s on-stage personas, marrying a sullen keyboard player and theatrical singer. “There’s myself, the guy from the Pet Shop Boys, and Duran Duran,” Clarke says in the film. “We’re all miserable fuckers; it’s a look which we just stole from Sparks.”

The band continued to knock out albums until 1988 when they paused for the failed Tim Burton project. During that run, some landed with a punch, such as the wonky pop of Angst in My Pants (1982), whereas others flailed and confused like Pulling Rabbits Out Of a Hat (1984). “Some might say Sparks are not as big as they might have been but in what terms do you equate bigness?” asks Russell. “Bank account? Record sales? Or the biggest amount of creativity you’re putting into music? In that respect, Sparks is successful. We have a body of work of 25 albums we’re really proud of. That’s success. And if it works commercially then great, because every artist wants to be adored by more people. If it doesn’t, at least we have our integrity intact by doing something unique in pop music. I’m proud of our determination and resilience.”

Further milestone records followed, from their 1994 comeback album Gratuitous Sax & Senseless Violins, and its triumphantly shimmering electronic pop hit When Do I Get to Sing ‘My Way’, to the boundary-pushing neoclassical chamber-pop of 2002’s Lil Beethoven, and on to their 2015 collaboration with Franz Ferdinand as FFS. Plus there was the mind-bending 21-night Sparks Spectacular in 2008, where they performed all their albums in full and in sequence, and they are still releasing excellent music via recent LPs Hippopotamus (2017) and A Steady Drip, Drip, Drip (2020).

Key to their longevity appears to be a sincerely harmonious working relationship – notoriously tricky for siblings in bands. “I did ask ex-band members if Ron and Russell ever fell out,” says Wright. “The closest to them approaching a bit of moodiness was over one of them having to go to the shop to get batteries. They’ve found a way to work that is beautiful. In the six years I’ve known them I’ve seen Russell without Ron twice but never Ron without Russell. They have separate houses but in my head it’s like Bert and Ernie from Sesame Street.”

Routine is also paramount. They wear the same clothes every day, eat the same lunch, do the same walk, go to the same cafe and they work from 10am to 7pm six days a week. “I’ve never seen Sparks off the clock,” says Wright. “They are so dedicated, it’s this slightly monastic pursuit.”

The brothers credit contentment with their respective artistic contributions. “Our roles don’t overlap,” says Russell. “Neither of us wants to be doing what the other is. That’s a huge part of conflict with other bands. Also, we have an unspoken vision, like a cause or a mission, to spread word that pop music can still be done in a fresh way. That shared vision really keeps the bond together.”

The pair retain an unflinching love for their medium even into their 70s. “Being childlike is a positive,” says Russell. “If someone says we’ve never grown up I say: you’re right.” Ron also credits a disdain for banality as key to their ceaseless creative spark. “We have hostility towards a boring approach to something sacred like pop music,” he says. “It’s motivating to be hostile to the status quo. I like watching sports, and sometimes athletes invent an enemy to be really motivated and come out and just kill. There’s that motivation of us versus them, even if you don’t know who ‘them’ is. That’s important because it means you’re always trying to throw something in people’s faces. You’re going to hear this, whether you’re interested or not.” They are, of course, already working on their 26th album.

The Sparks Brothers is in UK cinemas from 30 July. Annette will be released in the UK on 3 September.