Gangster films invariably play with fire, in the way that they glamorise the criminal world, seducing us with the dangerous charisma and fancy trappings of their anti-heroes. Martin Scorsese is the most obvious example, with the strutting, beautifully dressed Mafiosi of Goodfellas and Casino. The key to getting away with it, which both Scorsese and Francis Ford Coppola (in The Godfather films) always managed, is to ensure we see the emptiness and rot beneath the shiny surface.

This balance is even more imperative when dealing with real-life gangsters. And yet this new film about the notorious Kray twins, who cast a dark shadow over London in the 1960s, doesn’t seem all that bothered about it. Even the title of the film is begging us to like the Krays. And, courtesy of Tom Hardy’s tour-de-force performance as both of the twins, to a certain extent we do.

It makes for a strange experience, enjoying a film while knowing that it’s being kind to monsters. It’s a problem that prevents Legend from entering the top echelon of gangsters movies.

To be fair, writer and director Brian Helgeland is both following – and highlighting – a tradition of being nice to the Krays. At the height of their powers, they hobnobbed with the rich and famous, were treated as celebrities themselves by the tabloid press, were photographed by David Bailey and were allowed to get away with murder (literally) by the Establishment, until it became impossible to turn the other cheek.

Whereas the 1990 film The Krays focused on the relationship between the brothers and their mother, Helgeland opts for a different woman in their lives, Reggie’s wife Frances (Emily Browning). It’s her voiceover that narrates the film, and her difficult marriage that provides its through-line – from touching courtship, through marriage and her failed attempts to make her husband go straight, to her increasing marginalisation as the darker Ronnie exerted a stronger influence over his brother.

Helgeland is best known as the Oscar-winning writer of L.A. Confidential, which also covered the uneasy alliance between crime and celebrity. His script for Legend is nowhere near as rich. Though sprinkled with cracking dialogue, it spends too much time on Frances and in the West End clubs where the brothers showed off their ill-gotten gains, while being far too light on the extent of their villainy.

He’s better with the differences between the brothers. Reggie is presented as smart, charming, with matinee idol looks, the brains behind the partnership. Ronnie is probably a psychopath, certainly diagnosed paranoid schizophrenic, a volatile, violent man who relishes his identity as a “gangster” and will prove to be his brother’s undoing.

The technology involved in bringing Hardy’s two characters together in the same shot is impressive. The film’s most bravura scenes involve the twins in fights, first taking on a rival gang, then in a fabulously over-the-top bust up with each other.

But what’s more remarkable is the performance – or performances – the way that Hardy (with the help of the make-up department) can mould his features to play notionally identical twins whose appearances have been differentiated by personality. In contrast to the handsome Reggie, Ronnie is a cigar-chomping bull with a fish’s ever-open mouth. How Hardy manages it I have no idea. But the duality reminds me of the trick that Jeremy Irons pulled off as the twin gynaecologists of David Cronenberg’s Dead Ringers, leading us to forget that we’re watching the same actor. But whereas Cronenberg’s horror film is a genre masterpiece, Legend is, despite being entertaining, rather run of the mill.