“Dear Osama,” begins the unnamed narrator, a South London slapper (her word) played by former “Dawson’s Creek” star Michelle Williams. “They say that life is what happens when you’re busy making other plans. Do you find that, sitting in your cave with your Kalashnikov?”

Sadly the question is left dangling along with her dipthongs – so far as we know, Osama hasn’t deigned to reply, always assuming Michelle’s letter actually found the right cave (which is more than Morgan Spurlock manages in his new documentary: “Where in the World Is Osama Bin Laden?”).

Last year, Sundance served notice that Iraq would loom large over American filmmaking with the emergence of several important documentary and dramatic movies. This year, Incendiary is one of the few pictures to square up to the War on Terror, but I suspect it will prove as unpopular with British audiences as Redacted and In the Valley of Elah have proved with the Americans.

There’s something queasy and disturbing about watching violence visited on your country in a way that feels so close to home, and yet so fraudulent. At least a monster movie like Cloverfield or 28 Weeks Later gives the imagination room to roam; here, we’re more likely to be offended by the liberties the filmmakers take with urban geography, or the sheer tastelessness of the “cemetery in the sky” – a flotilla of blimps emblazoned with the faces of the dead moored across the London skyline.

Chris Cleaves’s novel attracted some notoriety when it was published just days before the London bombings of July 2005. It’s the story of a young mum whose husband and four year old son are killed when suicide bombers wreak havoc at an Arsenal v Chelsea match. As if that wasn’t bad enough, mum’s engaged in extra-marital nookie at the time with sexy "Express" journalist Jasper Black (Ewan McGregor).

With Jasper’s help she evades police roadblocks and gets far enough into the stadium to wind up in hospital herself. While the country mourns the dead, Jasper identifies one of the bombers and passes on the address of the killer’s widow and son to his scarred lover.

The stage seems set for a creepy revenge thriller built from the racially inflammatory suspicions and resentments running through contemporary Britain – but writer-director Sharon Maguire (Bridget Jones’s Diary) has other ideas. Bizarrely, Incendiary has at least as much in common with the chick-lit Helen Fielding inspired as PD James-style psychological suspense. Bridget Jones’s Breakdown is more like it.

Williams is barely out of hospital before anti-Terrorist police boss Terrance (Matthew Macfadyen) is cosying up to her bearing invites to a caravan show (cue for a heartfelt but deeply dodgy analogy between global terror and the sanctity of a man’s caravan).

“I’ll pick up the broken pieces of your heart and put them back together,” he vows, “No matter how long it takes!”

Is Michelle better off with flash Jasper (but can she ever forget that Arsenal game?) or boring, dependable Terrance (but hang on, isn’t he a Chelsea supporter)? More to the point, why should we care?

A gifted and versatile actress, Williams gets the accent right but she’s too clean-cut for a working class lass – you only have to imagine the edge Samantha Morton, Sally Hawkins or Nathalie Press might have brought to the part, a raw vulgarity that’s simply not in Williams’s DNA.

But there’s so much in this ambitious/opportunistic effort that misses the mark, from the one-dimensional characters to the craven plotting and sentimental tone… Maguire is playing with fire, and it blows up in her face.

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