Sharon Maguire, the director of Bridget Jones’s Diary, presented this strangely uneven film at the London Film Festival before its release. It has the old Arsenal stadium ripped apart by terrorist bombs during a match. A young mother who remains unnamed (Michelle Williams) lives in a tower block nearby and loses her husband and son (Nicholas Greaves and Sidney Johnston) in the blast.

She is distraught about her son, but not too bothered about her husband, a gloomy bomb disposal expert. After they had set off for the match she allowed herself to be picked up in a pub by Jasper, a tabloid journalist (Ewan McGregor), and they are going at it with a will when the bomb goes off.

Jasper, who is fascinated with her Cockney “directness”, is determined to find the perpetrator of the deed. In fact, one of the terrorists turns up at the memorial service for the dead at St Paul’s, and has a son (Usman Khokhar) whom the mother incredibly meets and befriends.

Added to all that, Matthew Macfadyen plays a high-ranking police officer, head of the terror unit, who, despite the disaster, apparently has time to pester the distraught mother and even take her away to his caravan by the sea.

Jasper tries to keep in touch with his lover but she, naturally enough, now resists. And there is an extended sequence when she imagines she is still with her son in the flat.

There are so many unlikely things in Incendiary that its competent direction, decent acting and eerily relevant subject matter count for less than they should. Williams does well under trying circumstances, giving her grief about her son the ring of truth. She’s better, at any rate, than McGregor, who glides through his part on autopilot.

Maguire, who also wrote the screenplay, taken from Chris Cleave’s book (which came out on the morning of the 7/7 London bombings), never manages to convince us that the plot holds together.  

While individual scenes work well, the whole simply defies belief. And, because of this, the final polemic which suggests that London, constantly renewing itself, will never be defeated, simply appears hopelessly sentimental.