So what's the big idea then?

Film-maker Sara Dunlop says her friends slag her off for doing adverts. But Big, her minute-long parody of the Barclays Bank TV campaign, is no mere sales pitch - it's up for a film awardher minute-long parody of the Barclays Bank TV campaign, is no mere sales pitch - it's up for a film award

Sara Dunlop's film Big lasts for 60 seconds and is a "response" to a certain, rather infamous ad for Barclays Bank. Johnny, a geezer with limpidly savvy eyes, introduces us to the big things in his life: mortgage, family, debts... In a series of tracking shots, we are welcomed into his grotty west-London world, until finally he floats like a ghost through a swanky foyer (he's so insignificant, the security guard literally doesn't see him), takes a lift and climbs up on the roof. He's ready for "the big drop" and it's at this point that any laughs to be had suddenly catch in your throat.

Sara Dunlop's film Big lasts for 60 seconds and is a "response" to a certain, rather infamous ad for Barclays Bank. Johnny, a geezer with limpidly savvy eyes, introduces us to the big things in his life: mortgage, family, debts... In a series of tracking shots, we are welcomed into his grotty west-London world, until finally he floats like a ghost through a swanky foyer (he's so insignificant, the security guard literally doesn't see him), takes a lift and climbs up on the roof. He's ready for "the big drop" and it's at this point that any laughs to be had suddenly catch in your throat.

Barclays must already deeply regret its "big is best" campaign - so lavishly directed by Tony Scott, so beautifully performed by Sir Anthony Hopkins - which just happened to coincide with the news that a number of the bank's rural branches were to be shut down. Suffice to say, it was a PR nightmare. And the success of Big surely won't help.

For the last month, this film has been shown in cinemas across London, as an advert for the homeless magazine, the Big Issue; it has also been entered in the Rushes Soho Shorts Festival, where it may well win The Switch2.net Short Film Award (it's down to the last three). The ironic icing on the cake is that Barclays is one of the companies sponsoring the festival.

Sara Dunlop has every reason to feel pleased by recent events. But who is she? Having watched Big, you would tend to assume she is a film-maker who a) knows something about displacement, and b) despises big business. And when this half-Chinese 26-year-old plumps down into her seat, you'd assume you guessed right.

The Big Issue didn't ask her to make Big; she and her friend Will wrote it; she shot it in a day, then offered it to the magazine free of charge, as a "political statement". She says if she wins an award, she wants Barclays to make a donation to the magazine.

Tony Top Gun Scott is not one of her favourite directors (nor does he seem to be much of a fan of hers; the company behind Big sent him a copy of the ad, but it was sent back, without comment; his people said he "didn't have time" to watch it). She prefers Robert Altman, Hal Ashby and the two Todds (Solendz and Haynes).

Dunlop also has strong opinions about good and bad behaviour. She has nothing but scorn for Tim Roth - who, along with Hopkins and Nick Moran, stars in the bona-fide Barclays ads: "Even the thought of The War Zone is exploitative to me, that's why I wasn't surprised he agreed to do the ads. He's such an opportunist."

Then there's Spike Jonze: "His dad's one of the richest men in America. I remember reading about how he'd trashed two cameras and thought it was really funny - I thought 'it's alright for you to have a gung-ho attitude. What about people who don't have that safety net?'."

And she positively has a fit about the fact that a friend of hers who works on the Big Issue has left for the Daily Mail: " The Mail is the worst, the worst... how could he?"

And yet, shock horror, Dunlop herself hails from a middle-class background, (social-worker father, teacher mum, convent school, college, that sort of thing). Worse still, she not only likes a lot of ads - she makes them. As her CV boasts, she's worked "for some of the biggest names in advertising", including Coca-Cola and Reebok.

It turns out she went to the extremely big advertising agency, Bartle Bogle Hegarty, for guidance with Big, noting, without a blush, that she shot the "big business" foyer scenes in the BBH building. And she's more than happy to acknowledge the input of BBH creative director Rosie Arnold.

"Rosie wanted us to cut out a line about Johnny having a big drink problem, and being caught with a big bag of Charlie - she said that would make the audience lose empathy for him."

You could argue that this is Daily Mail logic - keep victims respectable, people on drugs have only themselves to blame! - but as far as Dunlop is concerned, it just means she has kept everyone happy. She says that, as someone involved in helping companies to sell products, she understands compromise. "I'm not a Tony Kaye - I don't want to go out and bamboozle everyone and run riot with a client's product."

So Dunlop's not quite the urban terrorist one might initially take her for. Perhaps she's even, dare one say it, a trifle opportunistic? But is that such a bad thing? It may be that the crafty world of advertising is a fine place for film-makers to learn their trade - Alan Parker, Adrian Lyne, Tony Kaye, Ridley Scott, his brother Tony... they've shown there are worse ways to start. And, of course, the Rushes festival (at whose awards ceremony Parker will be handing out prizes) clearly aims to show that adverts can be judged as works of art.

Dunlop is relieved this should be so. "I've got a lot of friends who slag me off for doing adverts," she admits. "And one of my best friends from college (Dunlop did a film course at the University of Westminster) refused to go into anything commercial."

She pops a cigarette into her mouth. "But I've just seen his self-confidence go down and down. It takes years to write a script and in the meantime you're working in video shops and bars, slowly feeling worse and worse about yourself because you're not actually filming anything."

She considers that, thanks to her work in advertising, she's now ready to do a feature. She's almost finished writing a script with a friend, which she says will not be a typical "British" product. "I have a problem being inspired by Britain as a place to make films," she sighs. "Could you name five good, popular British films?" She chuckles: "I've set this new film in LA - that's my way out of the whole British film problem."

This may sound ominous to those who think LA is all about "bigness". But I'm not sure we need worry just yet. Before leaving, Dunlop tells me a bit more about her family.

Until recently, the only religious person in the family was her mother, who is a Chinese Catholic. But then Dunlop's father decided he wanted in on the act. Not only that - he wanted Sara to be his godparent. "It was so weird," she says with a happy grimace. "I had to stand in the church and say I'd guide him on his spiritual path and I whispered to him 'you know, I've strayed off the spiritual path myself a few times' and he just laughed and whispered back 'I know'."

She grins: "My boyfriend's been really horrified by the whole thing, he's really atheistic. And when he goes into the history of the Catholic Church, I can see it's really bad. But if you look at the things my parents do for the community, at the little place where they live and the little church they go to, you just can't slag them off." Little and big. Dunlop's not even aware that she's back to the idea that small is beautiful.

In 20 years' time, I wonder if Sara Dunlop will be making exciting, ground-breaking films about subjects normally deemed "insignificant"? Or big movies, with big stars, about absolutely nothing at all? You'd have to say the odds were in her favour. And ours.

The winners of the Rushes festival will be announced on 3 August