Filming on the Sly

EPPING FOREST, 1983. ELEVEN-year-old Garth Jennings and his friends have just watched a pirate copy of First Blood, Sylvester Stallone's debut outing as one-man army, John Rambo. Minds suitably blown by someone taking on 200 men with only a knife, and sewing up his arm without crying, they decide to make their own version using Garth's dad's video camera, some improvised props and plenty of imagination. Thus is born the legend of … Aron: Part 1

"Aron was the name of our Rambo character," explains Jennings. "I played the head of the Military Defence and it was my job to train our hero. In the midst of training, I'm captured by the PLO and they demand £5 million otherwise they're going to kill me. But our Rambo character hears about this on the news, comes after them, shoots everyone and burns the terrorists alive."

Jennings flashes a wry grin. "It was pretty amazing. Of course, it's only afterwards you go, 'What were we thinking?' "

Fast-forward to March 2008 and Jennings is running through his crude film-making beginnings as he sits on a barge in the East End of London. The barge is the creative hub of Hammer & Tongs, the video production company he runs with producing partner Nick Goldsmith, which counts among its output some of the most innovative music videos of the last few years. Latterly, though, it has been a base of operations for Son of Rambow, Jennings's next film after The Hitchhiker's Guide to the Galaxy.

Set over the course of a balmy summer sometime around the release of First Blood, the film channels Jennings's childhood experiences into a heart-warming coming-of-age tale that captures that pre-digital era when smelly rubbers and Space Dust were all the rage and budding film-makers dreamed of getting their homemade movie epics shown on BBC after-school show Screen Test. It's the story of Will (Bill Milner), an imaginative but sheltered child from a Plymouth Brethren religious sect (that part's not autobiographical, though Jennings did live next door to such a group) whose introduction to movies comes courtesy of a knock-off copy of First Blood that he inadvertently sees via his rebellious, misunderstood classmate, Lee Carter (Will Poulter).

An impressionable child, Will is inspired by the film in ways he never dreamed of and develops a touching, complex friendship with Lee as they make their own version, called, inevitably, Son of Rambow. Their hugely ambitious action movie features everything from French foreign exchange students to flying dogs to lunatic explosions. It's a paean to unfettered creativity and the positive impact movies – even ones around which there has been moral panic – can have.

"That's what was nice about doing this film," Jennings says. "Just remembering how when you're that age you don't worry about the consequences. If you like it, you just do it."

Sylvester Stallone himself, who gave the project his blessing early on, raved about the end result when he was in London promoting his own Rambo movie last month. Scheduling conflicts meant they didn't get the chance to meet and although Jennings admits to not really liking Stallone's latest – "it's a bit hideous, really, all that maiming of women and children" – he can't help but like the man and can't quite believe his film is coming out on the back of a new Rambo one. "It was pure coincidence. We first came up with the idea eight years ago."

Indeed, Son of Rambow should have been Jennings's first film, but he was offered Hitchhiker's out of the blue, which meant shelving it for at least another two years. The idea was that the sci-fi comedy's success would make Rambow easier to finance. Or so Jennings thought.

"We'd been led to believe that if you have a number one film in four or five countries there would be this chequebook somewhere and people saying, 'Well, what do you want to do next, guys?' And it was the polar opposite."

Jennings has no axe to grind but his experiences do seem to reflect a lack of vision at the decision-making levels of the British film industry, which tends to favour uninspired Full Monty clones over originality. "I guess there's a certain logic to that way of thinking, and we were asking for nearly £4 million. But at the same time I was like, 'I don't get it.' Anyway, all the money came from France in the end."

Which is a shame, really, as it's one of the best home-grown efforts of recent years. "I think it's the best film to come out of this country in a decade," says Jessica Hynes, who plays Will's Plymouth Brethren mother, Mary.

Nestled below deck in a neighbouring barge, the actress and writer – still better known by her maiden name of Stevenson – is adamant that Son of Rambow is the best thing she's been in.

"I just love it completely and feel very lucky to be part of it," she says. Playing a widow trying to deal with conflict that arises from her duty to her religion and her duty to her son's happiness, Hynes is underselling her contribution somewhat: she provides the emotional weight that helps ground the film's more whimsical flights of fancy. Still, it was the boys' story rather than her part that made her want to do it.

"I love the way that Rambo inspires them in such a sweet and gentle way. They sort of see in him the noble qualities of manhood that are possibly lacking in their own lives because neither of them has a father figure."

It's not hard to understand why this aspect appealed to Hynes. The Brighton-raised, National Youth Theatre-trained actress shot to prominence alongside Simon Pegg as the co-creator and co-star of cult British sitcom Spaced, which understood the importance of pop culture in shaping people's lives. "We were just swamped with American television and films growing up in the 1980s and 1990s and it did inform how we saw ourselves and how we saw the world," she says.

Ironically, that show is about to be exported to America – though without the blessing of its creators. "It's a shame they didn't ask us," she says, archly, of the production company, Granada. "Legally, they didn't have to, but it's pretty insulting … We made a programme that has made them hundreds of thousands of pounds and they sell it to America without even telling us …"

She can console herself with the fact that Spaced continues to inspire a devoted following, though its success did have an odd effect on her. While Pegg and Spaced director Edgar Wright went on to take Hollywood by storm with Shaun of the Dead and Hot Fuzz, Hynes says she lost confidence and "nose-dived a little", losing interest in a script she'd been developing, and starring in critically slammed sitcom According to Bex. "I felt quite lost, actually. Simon and Edgar had ideas about what they wanted to do next, and they didn't involve me. I also had two young children and was a bit at sea with that. For some reason it was a dark time. I can't explain it, it just affected all aspects of my life, including the creative part, and I made some really dodgy career decisions, because I was a bit all over the place."

With Son of Rambow opening and two screenplays in development, Hynes is adamant that she's back – but under no illusions about how difficult it is going to be to get her films made. "It just takes so long. Son of Rambow took (Garth] nearly ten years! So God knows. Hopefully it won't be as long as that."

For Jennings, though, this chapter of his life draws towards a close when the film goes on release this Friday. Whatever happens, at least he knows the film connects with audiences. Its premiere at Sundance secured it a $7.5 million distribution deal and it was a big hit at both the Glasgow and London film festivals. I tell him that at its London film festival premiere I spotted a burly security guard wiping tears from his eyes during the film's unexpectedly moving finale. Jennings roars with laughter. "Oh, that's brilliant. I feel like I've done my job if that's happened."

But did he ever think that such an inauspicious start –running around Epping Forest making his Aron: Part 1 – would lead him to this point? Apparently so. "I just decided by the age of 11 that that was it … It's amazing the ripple effect of watching Rambo."

• Son of Rambow is released on 4 April.