Out on the edges of metroland in Loughton, Essex, along the traditional escape route for East Enders bolting from tuberculoid tenements to roomier suburban homes, lies a strange and little-known place called Lippitt's Hill Camp. Black cab drivers get lost looking for it. When they do, it's hard for them to call for help: the thick foliage of nearby Epping Forest causes mobile phone signals to sputter and die.
The Camp is recessive by nature. It shrinks from publicity. But it has a history to snag the imagination of all those with a taste for the subterranean and covert. During World War Two, and right up until 1948, it was home to hundreds of Germans and Italian prisoners of war. A statue inside its iron gates marks their stay: "cut out of concrete by Rudi Weber 540177 while POW at this camp October 1946". Later, the Royal Artillery Anti-Aircraft Operations were based here. Underground bunkers and gun pits are still visible.
These days, Lippitt's Hill is used as a base by the Metropolitan Police Air Support Unit. Its officers are conquistadors of the London night. Each evening two crack teams of men and women set off from here and fly by helicopter across the capital. They're edge chasers, hurtling towards those spaces where ground officers fear - or are taught to fear - to tread: rooftops, railway lines, river banks. Their 500k machines, brimming with state-of-the-art hardware, allow them to scan the sprawling city, navigate it at great pace and elevation, and tail anyone who tries to carry out crimes under cover of darkness.
These avian police see, not just a side, but the entire face of London. The rest of us, victims of gravity, stranded down on the ground, have to make do with squinting from the windows of EasyJet planes or going for an evening ride on the London Eye. They do a job many of us can only dream of doing. Many of them, when they started out as beat bobbies, dreamed of taking to the skies too. Sometimes, after they had just been gobbed at by junkie-pimps they were trying to arrest in Camden, or as they stood on Horseguards Parade in full tunic and helmet with the midday sun turning the backs of their necks scarlet, they would hear the distant roar of overhead helicopters and think how nice it would be to be winched up.
The avian police see themselves as regular officers; before they begin their 7-7 shifts they sit around in the barracks watching European Championship football on Sky or reading authorised biographies of Ronnie Barker. But they are a select bunch: out of 30,000 PCs in the Met, only eighteen of them work in this Unit. As soon as the first call comes in, they leap into action, transformed by circumstances and technology into helmet and harness-sporting redeemers of the night's blackness.
They become Supermen. Their helicopters give them exceptional powers. They can zoom all the way across the congested city in less than ten minutes. High-power lenses and thermal imagers allow them to make out the crocodile logo on a clubber's T-shirt from 2000 feet in the air, read number plates in the dark, look through the windows of Canary Wharf and spot canoodling office workers from eight miles away.
They can fly within fifty feet of Big Ben and cause global repercussions by shutting down Heathrow Airport. They can light up the ground beneath them by shining 30-million-candlepower 'nitesun' torches, and help to rescue suicidal young women and befuddled old people who have got lost in Hampstead Heath. Amateur crooks who have caught the odd episode of The World's Greatest Police Chases and think they know the principles of heat-imaging hide under trees rather than, as used to happen, lie flat on fields: still they get detected.
"What's the most beautiful thing you've seen at night?
Oh, where do you begin? The mist lying in the valleys takes your breath away. The orange glow of the breaking dawn. Or sometimes when there's a full moon you can see its reflection in the Thames..."
The streets of London are made of gold. But only at night time and only from the sky. They lie there, glimmering like a Hatton Garden window display. Jewelled necklaces winking at us. At Piccadilly Circus and along Oxford Street the refracted neon gives them a ruby-red and emerald-green lustre. "Cracking night, Sukhdev," pipes the pilot, but I am too awestruck by the city's beauty to reply. This is the panoptic sublime.
The helicopter flies in orbit. It waggles and tilts. At times it feels as if the pilot has lost control, an outdoor tightrope walker about to fall to earth. The stomach-nausea is accompanied by bursts of landmark glee: there in the distance, is Wembley Stadium with cranes and machines perched over it like basketball players primed to slamdunk; there the Fabergé rugby ball of the Swiss Re Tower.
Politicians and demographers often assert that London is overloaded, crammed to the hilt, but from the sky it appears far from congested. The concrete jungle is nowhere to be seen. Even the most built-up areas are punctuated by large expanses of dark forest, empty parts of the city's night-canvas. The capital is an endless origami unfolding, stretching out horizontally rather than vertically. Its residential buildings are so crabbed and timid that any that stick out appear both heroic and lonely.
The sky is constantly lit up as private planes carrying Russian billionaires to vital soccer fixtures start their descent, and a whole queue of commercial airliners begin stacking to come into Heathrow; the effect is that of a corporate quasar game as lights continually strafe the darkness. Things invisible at ground level suddenly rear into view: industrial parks - there seem to be hundreds of them. And while, even from the Primrose Hill or Greenwich Park, the city melds into one largely unindividuated flatscape, at night time it becomes more composite in character, a loose and disconnected set of Lego pieces. One pilot describes Croydon as "an oasis of high-rise buildings, sitting there like downtown Dallas."
"Do I like night time?
Of course. When I was a bobby I used to go to the top of this tower block that had Big Ben in one direction and Tower Bridge in the other. At 3.30 am, 22 floors up, you could hear it strike: for that few minutes London was quiet and you could hear the birds singing."
The avian police, like black cab drivers, rely on The Knowledge. Spatial awareness is crucial. They can't afford to be disorientated even though they may have had to spin around a trouble spot half a dozen times and at different speeds and altitudes. New moving map systems allow the back-seat navigator to type in any address or postcode into a machine and, at the press of a button on the camera controller, have the camera pointing not just in the approximate vicinity of their destination, but at the precise house number or floor level.
It helps them to be as precise as homing pigeons. Yet, with single Global Positioning System screens able to accommodate only a quarter of an A-Z page, and the threat of computer malfunction an ongoing possibility, few officers have thrown away their guide books. They carry customised cartographies, special versions of the Collins Atlas in which traditional features have been overlaid with police boundaries and London letter-codes for all the police stations. They each supplement this information with biro'd squiggles of their own, creolizing the official geographies with private mnemonics for particular parts of the city. The lights near Wormwood Scrubs resemble a dog bone.
Before the moving maps were introduced, pilots learned to navigate London by its dark areas. Well-known landmarks were often obscured by fog, so they were forced to become airborne sensualists, feeling out and alert to the shapes and contours of forests and verdant areas. Victoria Park, it was generally agreed, looked like a boot.
"What's it like in winter?
It can be like being inside one of those snow balls. The snow comes at you from every angle. White dots spitting at you. If you watch Star Trek, it's like going into warp drive with all the stars coming at you. You don't experience snow like that normally - travelling at that speed and horizontally."
It's overcast tonight. The clouds we skim and fly through are disorientating. They make it seem as if smoke is rising, as if the city is ablaze. We hover briefly above a mist-obscured St Paul's Cathedral and for a moment I feel I have been transported back to World War Two and the scene of that iconic photographic of Blitz London in which Christopher Wren's dome is surrounded by acres of bloodied devastation.
The helicopter's thermal-imaging cameras irradiate the city. It looks skeletal, postmortemed. On the screen, its nocturnal hues and tints are reduced to black-and-white heat traces. Bleached and decoloured, it has become furtive, like a Customs' x-ray of immigrants smuggling into the country in the back of a lorry. The cameras induce suspicion: why has that snake of light suddenly concealed itself? - actually, it's just a train that's entered a tunnel. Every moving vehicle, at least initially, appears to be a portable terror-container, a nuisance bundle to be monitored and tracked.
The attack on the World Trade Center and now the suicide bombings in London have led to the avian police being placed on constant alert. Certain heli-routes that fly near key financial and political institutions are almost out of bounds. Each day the pilots are supplied with security updates which assess the threat from terrorists to US and Israeli 'prominent and representational interests'. One pilot describes his colleagues below as "ground troops". The thermal imagers themselves, though they're designed to help the police protect the city, produce images that resemble Baghdad, Vietnam - bombing zones for Allied troops. For a moment, London's nocturnal beauty vanishes: the forests seem ash-charred, lit-up areas ghostly apparitions.
"What's the most beautiful thing I've seen?
After a storm - when the city looks so washed and lovely. When it's a misty night, you can just make out the tops of high buildings like Canary Wharf: they look like islands in the mist..."
The avian police have to listen to six radio channels at once, a non-stop, mid-air, crosstown traffic of police sirens, command-centre requests, breaking news about pick-axe-wielding Turks on rooftops, random bursts of white noise. But they are also soundscape artists who bring noise to night London, calibrating it to create minimal or maximal impact. Too much roar gives suspects sound cover to break windows or climb fences. Too little, and would-be criminals think they can do as they please.
It would be an exaggeration to call the police sonic terrorists. But on quiet nights without remand-centre breakouts or high-speed motorway chases, they fly out to patrol London's crime hotspots in poorer boroughs such as Brent, Tower Hamlets and Southwark. When they see clumps of youths hanging around, they make the equivalent of a handbrake turn in the air. The blades cut the air harder. There is a loud thumping and chopping sound and everyone looks up to see the word 'police' on the underside of their machines. "They've all watched Air Wolf and think we can see through walls," laughs one pilot, "We're not going to tell them otherwise."
"What do we see that no one else sees?
You can see everyone's swimming pools. There's also some absolutely outstanding roof gardens down in the City. Certain areas like Chingford have a love of chequered patios. You've got a run on orange and cream tiles in Barking and East London at the moment: a B&Q lorry must have been turned over."
Flying over a city, especially at night time, allows a brief glimpse of freedom. It is to be liberated from the stress and murk of terrestrial life. Towards the end of their shifts, as darkness slides almost imperceptibly towards dawn, the avian police start to fly back to Lippitt's Hill Camp. Their heads ache and their backs are sore, but though they're at a low-ebb physically, for a few minutes they relax a little and let their minds wander. They think of their families and of past loves. They look at the line of pollution that hangs above the city, so thick they could walk on it, and wish it could be disappeared. They look at the city twitching into motion below them and are touched by its fragility. How beautiful Hampstead looks as it rises out of the mist.
A pilot, his operational lingo replaced by dreamy reverie, reflects on his working life in London:
"When I was working on the ground I certainly didn't like the city. Quite the opposite. But everywhere's lovely from the air. Even the worst bits look good. Like King's Cross: I never noticed the architecture of St Pancras before - all the stations and the buildings are fantastic. To be honest, I'd rather spend more time in the air than on the ground. Whatever you see on the horizon you can go to. You feel like a giant because the world is smaller."