Toa Fraser’s real-life hostage thriller about the 1980 siege of the Iranian embassy in London (it skipped a cinema release and arrives on Netflix on 3 November) is thoughtful, well-made, with a couple of excellent performances – and just a bit dull. Told with not quite enough suspense, its pin stays firmly in the grenade as six armed men storm the embassy in Kensington, taking 26 hostages and demanding the release of prisoners in southern Iran.

Still, it’s satisfying to see Jamie Bell, who has sometimes seemed to be stuck in a limbo of man-child roles, properly grown up. Here he plays an SAS soldier leading a team of heavily armed, black-clad heavies called in as backup should talks stall with the terrorists, who are threatening to kill one hostage an hour. Mark Strong gives a smart, subtle performance as the brains to Bell’s brawn – the detective in charge of negotiations. He’s rather repressed in the stiff-upper-lip tradition but also empathic. (As he builds a rapport with the gunmen’s leader, you sense him becoming emotionally invested in a non-violent outcome.) Outside the embassy, BBC journalist Kate Adie (a distractingly stiff performance by the Australian actor Abbie Cornish) is reporting on the crisis, watched by millions on TV.

The action takes place over the six days of the siege, moment to moment, the clock ticking. It’s leanly told from multiple points of view, without lingering too much on characterisation (with the exception of telephone calls made by Strong’s detective to his worried wife – a role so thin it could win prizes for dullest female character of the year). We get a glimpse inside the corridors of power, too. The late Tim Pigott-Smith, in one of his final performances, plays home secretary Willie Whitelaw, who communicates the still relatively new prime minister Margaret Thatcher’s refusal to negotiate with terrorists at Cobra meetings – and her apparent lack of squeamishness at sending in the SAS and risking a bloodbath.

The best scenes involve the SAS. Bell is terrific – a combination of cocky swagger and perfect focus – and there are a couple of stomach-lurching moments when his adrenaline-pumped squad are poised to launch an assault on the embassy, only to be told to stand down with seconds to spare. Then it’s back to tea and a ciggie in front the snooker. Director Fraser was clearly taking notes during Tinker Tailor Soldier Spy and, as a period piece, 6 Days is nicely detailed – the interiors, clothes and cars all in shades of fag-ash brown.