Leon Butler had a dream and a screenplay. What he lacked was capital – a situation the same as thousands of aspiring writers across the world, from Battersea to Beverly Hills. Yet two years later, the former property developer stood on set watching Idris Elba and Gemma Arterton speak the words he had written. And he’d never even sold his script.

We’ll come to the ‘how’ shortly. First, the film. Initially entitled One Square Mile (hey!), 100 Streets consists of three disparate narratives located in the areas of Battersea and Chelsea. Elba and Arterton play a former international rugby captain and his estranged wife; Charlie Creed-Miles is a taxi driver jolted into an unexpected nightmare; and Franz Drameh’s youthful drug dealer wants to swap the streets for the stage. Over the 90 minutes, these characters “brush against each other” (Butler’s phrase) without ever quite colliding.

Butler cites an unlikely inspiration for the screenplay: “The ‘Big Society’ – Cameron wasn’t good for much but he was good for the idea of the Big Society, where people take ownership of their own communities.”

While some might scoff at the already archaic term, Butler’s lack of cynicism is endearing, and apparent on screen. One character coaches a schoolboy football team, another runs a half-marathon for charity, a third volunteers at Battersea Arts Centre; these are people involved in their community.

But then communal involvement is how the film was made in the first place. In early 2012, Butler faced the dreaded slog of any novice writer: trying to find financial backing. Rather than hawk his script around the studios, Butler decided to cut out the money men. Why plead for cash when he could raise it himself with a little help from his friends?

“I wrote to 22 colleagues – some in the City, some actors, some social people – to network my script and get it seen. The truth is you can write a great script, but no-one’s ever going to read it. So I used my contacts to network.

“For 18 months, this committee met every month. We held ten events, helped local charities and pursued the money. Within two years we were filming. We raised £3m.”

Every night I’d be thinking: ‘This could all be gone tomorrow – the trailers, the set, the cast. What am I going to do; my life’s over?’”

It’s an impressive example of initiative, and one that testifies to the enduring appeal of cinema. As Butler notes, “Everyone loves film, but so many people were keen to help us. Nobody had heard of someone doing property development and suddenly making a film.”

Backed largely by City money, Butler was nonetheless keen to involve local charities: “We didn’t want to come to the area, make a film and then bugger off.” While commendably philanthropic, this decision also proved his salvation. Shooting had already begun when one of the backers mysteriously vanished, along with his pledged money.

“We had a commitment from one European investor: £2m. Ten days into shooting that guy disappeared on me. My family and friends were on set saying how proud they were of me, and every night I’d be thinking: ‘This could all be gone tomorrow – the trailers, the set, the cast. What am I going to do; my life’s over?’”

Salvation came in the form of businessman John Caudwell, the co-founder of Phones4u. Caudwell’s charity, Caudwell Children, already featured in the film, and Caudwell himself had bought a minor walk-on role. Within a week the businessman had raised the necessary money and saved the production. Butler hasn’t seen or heard from his missing investor since.

The involvement of Elba, a genuine coup, came about through well-placed friends and a little bit of luck. One of the committee members knew Ross Hubbard, a prominent casting director. The script passed through Hubbard into the hands of Elba, who liked what he read. Initially Elba came aboard as a producer but agreed to take a starring role and be the ‘name’ required to get the film greenlit.

It’s a truth Butler readily concedes: no matter how innovative the creation, ultimately the onus always falls on the bottom line. “Unless you’ve got a star, no-one’s going to go and see it. You need a name.”

Elba is most certainly a name. Moreover, he enlisted another: his friend and former co-star Arterton, to play his wife. The pair bring a touch of Hollywood glamour to the cast; indeed, their celebrity nicely informs the social status of their Chelsea-dwelling characters.

Although Elba will be the name above all of the posters,100 Streets is very much an ensemble piece. Cheery cabbie Georgie (Creed-Miles) and the delinquent Kingsley (Drameh) share a similar amount of screen time; the latter is arguably the spiritual heart of the film. Kingsley’s friendship with Ken Stott’s elderly thespian – they meet when the former is completing a stint of community service – embodies Butler’s view of London as a city in which several different worlds exist within the same postcode.

“We’ve seen Kidulthood and we’ve seen Notting Hill. Those films are great but London’s not quite like that. Ladbroke Grove isn’t isolated from a street in Notting Hill, it’s next to it – but in those films they’re very much isolated. The kids in the hood, Hugh Grant in his bookshop – they’re not real. I wanted to show London in its reality. Everyone brushes past each other and lives in the same area.”

Even the most outlandish scene is inspired by real events. In 2008 barrister Mark Saunders, crazed on alcohol, began letting off a shotgun from his house in Markham Square. Armed police arrived and a siege ensued, with tragic consequences. All three storylines are linked by violence, some of it genuinely unexpected. (One particular incident arrives from out of nowhere and provoked exclamations throughout the cinema.)

Originally, 100 Streets contained four storylines but one fell victim to the cutting room. As Butler admits, “We had a three-hour film off a 110-page script.” The film now runs to a taut and distributor-friendly 90 minutes.

Butler wants his story to continue beyond the release: “We’re looking at a TV series that comes out two years later. Half the main characters from the film, half new characters. Much in the same vein of This Is England.”

Yet for the moment Butler has other projects on his plate, most intriguingly a romantic comedy set in Soho. It’s clear he won’t be abandoning his home city any time soon. And why would he? As the man himself says: “London is the capital of the world.”

100 Streets is out in cinemas on 11 November