Among the companies Ramsay's team approached for that scene were Coca-Cola and Campbell's Soup; both refused to sanction the use of their products in We Need to Talk About Kevin. Given its subject matter, their reservations were hardly surprising.

Ramsay's use of the colour red in the film (the opening sequence features Eva being splattered with tomato juice from the fruit thrown about at the annual La Tomatina festival in Bu?ol, Spain) serves as a metaphor for the violence to follow. Eva, a travel writer who gave up her job to raise her son, Kevin, has a fractious relationship with the boy which eventually explodes when he goes on a bloody spree (off screen) that tears apart their family and neighbourhood.

Having made her previous films in Britain, the Glasgow-born, London-based Ramsay admits she was unhappy with some of the 'stringent rules' she was forced to follow while shooting Kevin in the United States.

Restrictions on backdrops aside, she was infuriated that she couldn't include intriguing people she met while production was in progress. She was unable, for example, to give a keen and interesting boy a walk-on bit part in a Halloween scene because, according to union rules, she couldn't use unregistered actors.

'And the extras were walking so slowly to be in camera, it was horrible - that was frustrating sometimes,' the 42-year-old says with a growl.

Pre-production on We Need to Talk About Kevin was protracted: Ramsay and her musician-screenwriter boyfriend Rory Kinnear spent nearly four years writing and rewriting an adaptation of the 2003 Lionel Shriver novel, while BBC Films looked for financing for the project. In 2009 the production finally came into shape, with the casting of Tilda Swinton (who had then won an Oscar for her role in Michael Clayton) as Eva, and BBC Films securing the money to bring Ramsay's vision to fruition.

Ramsay's Kevin differs from the novel. Shriver relates the story through a series of letters that Eva writes to Kevin; Ramsay has largely eschewed this confessional approach to telling the story, choosing instead to sculpt what she describes as 'a montage' that interweaves different segments of Eva's life. It's evident right from the Tomatina festival sequence - which happens before Kevin is even conceived and Eva is a vibrant, free-spirited writer - and segues into the grim, equally crimson-hued present, with those tomato stains acquired in fun now replaced by the red paint vandals throw at Eva's rundown house. She is condemned to life as a pariah in the community because of her son's deadly actions.

'I'm not making easy statements [about motherhood] here, this film just poses some questions,' says Ramsay. 'She doesn't like the child, and the child picks up on that. I see it as kind of a Greek tragedy, or maybe even a perverse love story.'

The concerns of Coca-Cola and Campbell's executives aside, We Need to Talk About Kevin hasn't stirred controversy. Reactions have been mostly favourable since it went on the film festival circuit and started its commercial run around the world.

The film premiered at the Cannes Film Festival with positive reviews, and went on to win prizes both at home (best film at the London Film Festival, best director at the British Independent Film Awards) and abroad (a best-actress title for Swinton at the European Film Awards, among others). The only setback was probably in being shut out at both the Baftas (three nominations, no awards) and the Academy Awards (no nominations).

Such success has been a major morale boost to Ramsay, who has had to overcome many hurdles over the past decade.

Yet Ramsay has had a barnstorming start most aspiring filmmakers can only dream off. At 26 she won a jury award at the 1996 Cannes Film Festival for her short film Small Deaths, a graduate project at the National Film and Television School in London. She scooped the prize again two years later with Gasman.

More plaudits came her way in 1999 for Ratcatcher, a poignant, visually stunning film about a working-class boy's yearnings for connection (through friendship with a much-abused girl) and liberation (with magically surreal scenes showing the boy running in wheat fields and ambling through a half-finished housing complex) in a rundown part of Glasgow. For this, Ramsay was named best British newcomer at the Baftas.

So when she finished her next film, Morvern Callar, in 2002, she wouldn't have an inkling of the difficult times ahead of her. Having signed on to direct an adaptation of Alice Sebold's The Lovely Bones well before the novel became a literary phenomenon - she recalls working on a screenplay based on the unfinished manuscript - Ramsay was unceremoniously dropped when Steven Spielberg expressed an interest in the project.

New Zealand director Peter Jackson (The Lord of the Rings trilogy) eventually delivered the film in 2009; the reviews were almost universally negative.

'I was on The Lovely Bones for five years before Peter Jackson, and it's a big horror story for me anyway. It's quite a disaster,' Ramsay says. She had a drastic interpretation which would transform Sebold's narrative from the point of view of a murdered child to that of the girl's grief-stricken and increasingly distraught father.

Ramsay's idea was torpedoed by Spielberg's DreamWorks. 'I thought I was taking the story in one direction, but [the book] turned out to be a huge best-seller, on a much bigger scale than [I expected] when I picked it up earlier. I was about to make it with US$10 million or something, and I was working with the crew I worked with on Morvern Callar.'

Jackson's version, which cost US$65 million to make, took US$44.1 million in the US; only its international run saved the film from being a bomb.

Between Morvern Callar and We Need to Talk About Kevin, Ramsay's only production forays were music videos for Mancunian band Doves (Black and White Town, 2005) and Scottish singer-songwriter Tommy Reilly (Jackets, 2009), as well as an advertisement for the British pregnancy helpline Amber.

Her current success is a vindication for the filmmaker, who jokes that she only got by during those bleak years by 'being very poor'.

Fresh from Kevin's positive reviews, Ramsay says she is considering two projects: one is a venture into science-fiction territory, while another is a smaller production set in her hometown of Glasgow. Or maybe something combining the two, she says with a laugh. 'Psychological thrillers in space, maybe - who knows?'

We Need to Talk About Kevin opens on March 1