Returning home, third-class, after living as a down-and out in Paris, George Orwell fell in with a couple of Romanians and found himself praising his country’s many pleasures, from mint sauce and marmalade to the scenery and architecture. The book he wrote after spending time in London doss houses tells a different story. The utopian myths soon fade if you are poor or Other.

Ben Judah’s epic account of contemporary London is similarly motivated by a desire to show our capital in its true (new) colours: as a megacity of global migrants, some of them rich, most of them poor, few of them happy with their lot. Knightsbridge gets a chapter and so does Mayfair’s Berkeley Square, but it’s the people and places further out that really interest him – the Poles, Somalis, Afghans and Ghanaians in areas such as Beckton, Ilford, Edmonton, Catford and Harlesden. The ethnic majority, in other words: the 55% of London’s population that isn’t white British.

“I have to see everything for myself,” he tells us at the outset, “I don’t trust statistics.” An early chapter finds him dressed as a beggar and, Orwell-like, bedding down with 16 Roma in an underpass by Hyde Park. His companions complain long into the night: about the cold, the damp, the debt enforcers, the rich Arabs “coming in and out of the golden places” who give them nothing. They also complain about the police, so Judah takes himself off to Frontline Peckham – Peck’Nam as it’s called – to hear the story from the other side. With his mournful talk of the vanishing English and the rise of ghettos, the policeman he interviews might pass for a member of the BNP. But he is Nigerian, and the story of how he got to be where he is makes compelling reading.

Judah might not trust statistics but he weaves them into his narrative, and at the end – in case we don’t trust them – he gives the sources, mostly from government surveys. In 40 years the percentage of white British in London has fallen from 86% to 45%; 600,000 of those in London are there illegally; the number of Africans would fill a city the size of Sheffield; 57% of births are to migrant mothers. A gun is fired on average every six hours; 96% of London’s prostitutes are migrants, as are 60% of its carers. This is London, Judah insists: multitudinous and multi-ethnic. Or – less an assertion of truth than an expression of incredulity – This is London?

Some of what he uncovers makes for dispiriting reading: the fierceness with which each ethnic group sticks to its own enclave; the drug wars and protection rackets; the speed at which the inner city is being socially cleansed and “old immigrant London” pushed out to the fringes. But the book is reportage, not a moral tract; Judah wants to tell it like it is, with I-am-a-webcam neutrality, not preach or harangue. He hears things being said and, even if they are paranoid or semi-literate, faithfully sets them down: “Look what’s happening in London, mate … Zone 1’s being sold to the Russians and Zone 2’s being bought by the poshos.” “You know what shock me about the London. That you on your own. People not friendly … Some bad men come and mug you … and they will walk right past you.” “You want the same fucking Mo Farah wonder story … well, let me tell you what, you’re ain’t gonna find it.”

Judah is forced to lie to a few of those he interviews. I’m a Romanian immigrant, he tells the “spiritual healer”, AKA witch doctor, with whom he is negotiating to see an African prophetess. I’m conducting a survey for William Hill, he informs the tight-lipped gamblers in a Harlesden betting shop. I’m Russian, he says, to get a bed in a Barking doss house – which since he speaks Russian (his previous book was about the Putin era) is less of a fib. In general, though, Judah is frank about who he is and what he is up to. To persuade two sex workers in Ilford to talk about their murdered friend, Mariana, he’s able to show that his interest is neither casual nor prurient: he attended the trial of the man who killed her.

That interview takes place in a car and there’s a black-and-white photo to prove it (one of many scattered through the book). Elsewhere, Judah takes the N21 bus, among sleepy Africans en route to their 5am office-cleaning shifts. Mostly he walks, notebook in hand. The observations come thick and fast, and at times the detail is oppressive: there are too many sights, sounds and smells to take in, and the switches from journalism to a kind of prose poetry feel strained. You start to doubt him, too: how does he know that man is Spanish or that woman Vietnamese just by looking? Was the day he visited a betting shop really Grand National day and, if so, wouldn’t that make the clientele untypical? Is it true that every time a “Nan dies in Neasden her house gets passed over to a key-jangling landlord”? With a story as powerful as this, there’s no need to sex it up or add colour. It’s sociology, after all, for which Henry Mayhew is a better model than Martin Amis.

It’s when Judah sits down with someone and listens that the book really takes off. He is brilliant at getting people to speak: the London Underground cleaner; the Polish builder; the Egyptian heiress; the Filipina housemaid; the imam who washes the bodies of the dead; the teacher; the carer; the gang leader. Among the mass of migrant stories are recurring tales of the glamour of London as seen from afar, and the grime, fear, poverty and violence seen close up. We learn a lot about the work that migrants do and how they see the British. Mean, ugly, lazy, cruel, secretive and snobbish are among the words used about us, though there is respect for our constitution and amusement at how we’re always saying sorry.

In a few cases Judah gets so close to his protagonists that he writes as if from inside their heads, in the manner of WG Sebald. There’s the Polish registrar in Catford, for example, who describes her own love life as well as her job marrying couples and deciding which ones are genuine and which doing it for a passport. When she recalls being invited to a Nigerian-Polish wedding party, on “one of those lingering and beautiful fallings of light there is only in England in the summer”, she loses herself in rapture and he lets her run on as if he’s not there.

Judah’s too young, at 27, to feel nostalgia for the old London. Though born here, he has lived and worked elsewhere, and sees it as if from outside. “I don’t know if I love the new London or if it frightens me,” he says early on. If fear outweighs love in what follows, it’s not for lack of trying: at one point he practically begs the Nigerian teacher he talks to in Plaistow to tell him something positive about her pupils. It’s easy to imagine how Nigel Farage or the Daily Mail might exploit his material. But Judah resists drawing grandiose conclusions, preferring to leave us with an image instead: two boys – one Asian, one black – with their arms around each other in a Muslim graveyard. The sky is powder-blue and they’re singing. “Brotherly” is Judah’s adjective for them – a hopeful emblem to end a book in which the divisiveness of the capital (between ethnicities, between postcodes, between rich and poor) is all too apparent.

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