She’s got guts, Sharon Maguire. War-on-terror movies like Lions For Lambs are belly-flopping at the box office, so making her long-overdue second feature a tornfrom-the-headlines, anti-terrorism melodrama was one hell of a punt.

In addition, this tale of the tragic aftermath of a big-city bombing seems the polar opposite of her first film, the Chardonnay-swilling Bridget Jones’s Diary. But as it turns out, Incendiary is more like its dark counterpart, a grittily sentimental and unabashedly weepy ‘women’s picture’ that forces tears instead of cheers from its audience.

Once again, there’s a stunning central female performance, this time from Michelle Williams, seamlessly convincing and impeccably accented as a nameless chav slapper, neglected by her policeman husband but devoted to her little boy.

The latter pair die in a terrorist attack on the Arsenal stadium while she’s engrossed in sweaty sofa-sex with slick journalist neighbour Jasper (Ewan McGregor). Drowning in guilt, she develops an obsession with the young son of one of the suicide bombers.

Despite a punchy, horribly plausible start (the bombing and its aftermath is unnervingly realistic, a skilful CGI replay of 9/11’s giant falling debris and fleeing crowds), the movie can’t make up its mind whether it’s a thriller or a psychological study. Maguire obviously wants the storyline to reflect the fractured, nervy mindset of her characters and a city in crisis.

Yet as it bounces from Jasper’s desperate sleuthing into a Scotland Yard cover-up, to ‘Young Mother’s’ plucky, griefstricken letters to Osama Bin Laden, and then a tense police shoot-out, the plot becomes as scattered as its heroine.

Reducing its male characters (McGregor, Matthew Macfadyen) to two-dimensional onlookers doesn’t help either, as the movie gradually loses sight of everyone except Williams.

By then, Incendiary has become a tear-stained monologue about bereavement and resilience – it goes off with a bang, but ends with a whimper.