If Aeronwy Thomas is disappointed she is not portrayed in The Edge of Love, John Maybury’s moving and visually stunning film centred on the second world war antics of her father, the rambunctious Welsh poet Dylan Thomas, she does not show it. As a two-year-old, she was present in New Quay, west Wales, when the incident that gives the film its dramatic focus took place.

There, on the night of March 6, 1945, a family friend, Captain William Killick, repeatedly fired his machinegun into Majoda, the flimsy, asbestos-ridden bungalow overlooking Cardigan Bay where Aeronwy and her parents were living. She was asleep in a back room, directly in the line of fire, while Dylan and his blowsy wife, Caitlin, were carousing with guests at the front. It was a miracle nobody was killed.

Killick was consumed with a volatile mixture of alcohol, sexual jealousy and posttraumatic stress disorder after returning from a bloody tour of military duty in Greece. He found his Welsh-born wife, Vera, living next door to the Thomases. They had fled the dangers and adrenaline rush of London in the blitz for the peace of Wales, where their bohemian life-style had continued, while he was risking his skin.

On one level, he was unable to readjust to reality - a universal complaint of the returning warrior. Maybury does not spell out the parallels with Iraq, but they are always there. More specifically, Vera had been supporting the indigent Thomases from her husband’s hard-won army pay. To top it all, Killick suspected that she had been having an affair with Dylan – who, to give the story added drama (and justify its title), had supposedly been her first lover when they were teen-agers in Swansea.

Deviation from truth is a potential problem in any biopic. As the biographer of Dylan Thomas, I am not aware of any affair between him and Vera. As a boy, he had simply been friendly with her family, the Phillipses, who lived in one of the larger, more welcoming houses near his own in the Uplands area of Swansea.

Rebekah Gilbertson, 34, who conceived and produced the film, admits there is no evidence for an affair, but says: “We wanted to find the dramatic truth about the characters. The actual truth was scarce.” She has a personal investment: William and Vera Killick were her grandparents. She was, therefore, keen to hear Aeronwy Thomas’s reaction after a screening of the film in Soho last week, and was visibly relieved when this proved positive.

It might have been different. When I took Thomas round the corner for lunch at the French pub, one of her father’s regular drinking haunts, she told me, with the quiet humour that has made her an excellent ambassador for the Dylan Thomas estate: “I should be very angry with that man Killick, because he tried to kill me.” She bears no grudges, however, adding that this was only one of several times she nearly lost her life as a result of her parents’ rickety routine. Aeronwy is a poet who has recently returned from a tour of America, where she read from both her own and her father’s work, but has turned her back on her parents’ unpredictable, often emotionally damaging existence. Married with two children, she lives in New Malden, southwest London, where she worships at the local church.

With its obeisance to The Map of Love, a 1939 collection of Dylan Thomas poems, this film is about a foursome. For the purposes of the drama, Vera Phillips (played by Keira Knightley) is a sultry singer in wartime London. While being courted by Killick, she meets her supposed old flame, Dylan, who, when not getting drunk, makes propaganda films for the government. Vera develops a close friendship with the promiscuous Caitlin (Sienna Miller), which makes for problems when, after reluctantly marrying Killick prior to his call-up, she cannot get over her love for Dylan. Amid much frolicking, their fragile relationships are shattered by Killick’s return, the “Majoda incident” and his subsequent trial for attempted murder.

Aeronwy Thomas is amused by the film’s overt prettiness. “The dresses all look too expensive,” she says, recalling her childhood. “Where are the patches on the elbows of coats and cardigans?” She is happy, however, to recognise the aesthetic demands of commercial cinema and to agree that, while sometimes economical with the actualité, this exuberant film captures the spirit of her parents’ lives. She feels it is successful in evoking her mother, with her Irish penchant for truth, above all, and is not put out that the film shows her older brother, Llewelyn, rather than her, at Majoda. He was staying that night with her grandmother in Hampshire. When she raised this point with the makers, she was told this was a drama rather than a documentary. Gilbertson adds that it was necessary to keep the number of characters small and readily identifiable.

Thomas’s only beef is the portrayal of her father as a vindictive man intent on sending Killick to jail. In particular, Dylan gives evidence at the trial that the captain was sober, and therefore more culpable, at the time of the shooting. Aeronwy says (and evidence supports her) that Dylan in fact tried to help Killick. With a hint of weary experience, she adds: “People who drink have a code that anything that happens under the influence of alcohol is forgotten the next day. You can’t require people to be charitable to you if don’t reciprocate.” In the context of the film, this lapse is understandable. Dylan’s anger expresses the break-up of his happy, irresponsible three-way ménage, as Killick’s return forces Vera to choose reality over romance.

For Gilbertson, the movie’s release is the culmination of a process that started seven years ago, shortly after she won a place to study film production at the National Film and Television School. Even before starting her MA course, she was thinking about how to approach a module that required her to develop her own story. In October 2001, her beloved grandfather William died. Another family member suggested that she might look at his involvement in the “Majoda incident” for her project.

Gilbertson was intrigued. She was aware of this family history, but it was not discussed. This was due in part to Killick’s embarrassment at his moment of madness (he tried to stop the publication of a letter in which Dylan described the events), and partly to reaction against the poet in Nonconformist Wales, where he was regarded as a wastrel.

With the 50th anniversary of Dylan’s death looming in 2003, however, he was rediscovered and attitudes began to change. The actor Neil Morrissey bought up hostelries around the poet’s later home in Laugharne, Carmarthen-shire (including Brown’s, the pub where Dylan took his daily tipple or ten), and, fresh from success with Enigma, Mick Jagger’s company, Jagged Films, had a script for a biopic ready to run. Even Welsh tourist boards belatedly woke up to the potential of a home-grown cultural hero.

At the same time, Gilbertson’s career took off. On a parallel track, she won a fellowship from the National Endowment for Science, Technology and the Arts to make a short film about sustainable development. Her script consultant was the playwright Sharman Macdonald, best known for When I Was a Girl I Used to Scream and Shout; and playing a small part was Macdonald’s teenage daughter, Keira Knightley, who was just about to find acclaim in Bend It Like Beckham.

As a novice producer, Gilbertson needed someone experienced beside her. At film school, she had been taught by Sarah Rad-clyffe, who had produced My Beautiful Laundrette for Working Title, and who agreed to work on this project.

Where, though, to find a director sympathetic not just to this setup, but to the world of Dylan Thomas? Gilbertson gravitated to the cinematically adventurous John Maybury, with whom Knightley had worked on a feature called The Jacket. He seemed ideal, having enjoyed success with a biopic about Francis Bacon, another Soho habitué. Maybury, however, was notably elusive, even for the pulchritudinous star of The Jacket. So Knightley, who had committed herself wholeheartedly to the project, took to attracting him with poems. (Now those would be collector’s items.) Eventually, he agreed to read the script, though it is not clear if her verse or gifts of champagne did the trick.

Knightley’s involvement helped Gilbertson to raise £8.5m, attract other stars (Lindsay Lohan was once mooted to play Caitlin) and talk up the film with suggestions of a lesbian affair between Caitlin and Vera; these are not followed up, though plenty of bedroom romps remain.

Aeronwy Thomas takes this all in her stride. “There’s a nebulous area between fact and fiction, which can be dealt with rather badly,” she observes. “This film deals with it well.” She adds: “It’s a sort of utopia, really, with Dylan sitting in as an icon.”

Andrew Lycett’s Dylan Thomas is published by Phoenix (£8.99); The Edge of Love opens on June 20

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