Dressed in a somewhat conspicuous Little Red Riding Hood coat, sporting thick-rimmed crimson glasses and immaculate matching nail varnish, 22-year-old Lucy doesn't look like the kind of person who regularly breaks into derelict buildings just for the hell of it.
But, on an overcast autumn afternoon, she is scrabbling through the smallest of openings in a barbed-wire fence surrounding the now abandoned West Park mental asylum on the outskirts of Epsom Common, Surrey.
Crawling in the dirt on hands and knees, her 5ft 2in frame slips easily through the hole and briefly vanishes around the corner of the imposing asylum, before reappearing. "Come on, then," she says impatiently, "the coast is clear. You'll be able to fit through. Security haven't spotted this hole yet. Usually they board them up or cover them in anti-climbing paint that never dries."
The reason Lucy knows this is because she's been to this asylum many times before; seven times, to be precise. Her visits to West Park, which closed in 2003, were not as a patient but as a so-called "urban explorer", a shady group of adrenalin-junkie travellers, photographers and building enthusiasts who spend their time infiltrating the forgotten, forbidden corners of Britain's past.
Criticised by the Royal Society for the Prevention of Accidents as "dangerous and irresponsible behaviour", urban exploration – as those who partake like to call it – is becoming popular in Britain. It is also a growing headache for the security firms who police many of the sites that are routinely targeted. The main online forum for explorers has signed up 3,000 members since its creation in 2005, and up to 100 new members join every week.
Each weekend, groups of enthusiasts don exploring alter-egos and head for the myriad abandoned asylums, hospitals, factories and ammunition dumps that, unknown to most of us, are scattered around our cities and countryside. The scene is renewed by the regular addition of buildings and factories that are abandoned each month. Enthusiasts monitor local newspapers and radio stations for any sign that a place may have recently become unoccupied – a factory closure, a bankruptcy, plans to regenerate a site.
It may be illegal to investigate them, but such buildings are a fascinating, if dusty, window into a bygone era. The explorers use aliases to protect their identity, adding to the movement's mystique. Many are photography enthusiasts who post artistic pictures of their latest daring exploits online. Others are in it purely for the thrill, clambering to the top of towering cranes or exploring the network of sewers and storm drains beneath.
Some even see exploration as a politically motivated act, a way of sticking two fingers up at Britain's CCTV-monitored society or lampooning the health and safety ethos that seems to want to wrap society in layers of cotton wool rather than accept that life itself is risky.
To say that Lucy, an art student from the West Country, is obsessed with urban exploration would be something of an understatement. The satnav in her beaten-up Ford Fiesta is programmed to alert her when she's near abandoned buildings. She admits to driving up to 400 miles on a whim to explore rumours of the latest abandoned find.
Known to many of Britain's growing ranks of security guards by her pseudonym Rookinella, in the past three years she has clambered, crawled, climbed and broken her way into hundreds of abandoned and derelict buildings across Britain and northern Europe. Recent adventures have taken her into abandoned ammunition depots in Wiltshire, and to a former military hospital outside Berlin where Hitler was once treated. She also got inside the Big Brother house, where she says only one security camera was on the lookout for intruders.
West Park is one of the explorer fraternity's most popular sites, owing to its sheer size and eerily preserved interior. Built between 1912 and 1921 by William Clifford Smith, West Park was one of four great asylums built in Surrey at the turn of the 20th century to house and treat the capital's mentally ill. Like so many buildings that once served the National Health Service, the asylums now lie abandoned, waiting to be turned into flats and offices or simply demolished. Meanwhile, they are a tempting lure for intrepid explorers willing to break the law for a spot of very off-the-beaten-path tourism.
Even once we're through the security fence, finding a way into the maze of buildings isn't easy. Security guards routinely patrol the premises and hammer shut any open windows and doors they come across – which is why, on their way out, professional explorers always make sure they close the exits just enough for them to look as if they are impassable.
Most explorers try to avoid carrying equipment such as screwdrivers or crowbars, as doing so can land them in much greater trouble with the police if they're caught. Trespass is a relatively minor civil offence, but screwdrivers and crowbars can land you in a criminal court under much more serious breaking and entering charges.
Eventually we come across a window that our guide remembers is loose, and we climb through. The room is remarkable. It's as if time had suddenly stopped. Apart from the peeling paint, the mould and the dust, the room looks ready to be a fully functioning ward. The beds have been removed but the partition walls, curtain rails and bedside lamps – even the Peter Rabbit posters on the walls – have been left untouched. Zimmer frames, stretchers and bathing equipment litter the place.
"God, I love that smell," says Rookinella, her voice echoing down the cluttered corridors and wards. "It's the smell of dead hospitals, you never forget it. For me, it's like chocolate. I'm utterly addicted to it. If I get a scent of dead hospital, that's it, I'm off."
The exact origins of the urban exploration movement are unclear – the intrepid have always been tempted to explore the forbidden – but the origins of a popular urban movement probably began some 25 years ago in America when students at the Massachusetts Institute of Technology decided to start exploring a series of underground tunnels near their sprawling campus.
The term "urban exploration" is attributed to the Canadian architectural historian Jeff Chapman, who went by the name "Ninjalicious" while exploring his hometown of Toronto. Through the mid-Nineties, he published a magazine called Infiltration – "the zine about going places you're not supposed to go".
For would-be urban explorers, Infiltration was a treasure trove of information, advising readers on every conceivable tactic, from safely navigating storm drains to evading hotel security staff by dressing up in official clothing and hard hats to fool the guards. Chapman died three years ago from cancer at the age of 31.
Walking through wards that once housed hundreds of inmates, it's easy to forget that we are breaking the law. Each room is utterly intoxicating, offering up new and fascinating insights into what they were once used for.
In one small space off one of the many glass corridors that connect the various wards lie an assortment of abandoned dentist's tools and a chipped set of dentures. Another space is a former laundry room, complete with dilapidated washing-machines and dryers that look like they could leap back into life at the flick of a switch. The kitchens still house giant industrial extractor fans and rows of ovens.
But exploring abandoned buildings can be a dangerous affair if you don't know what you're doing – which is why those in charge of securing derelict sites are becoming increasingly concerned about ordinary members of the public making their way into places that may be structurally unsound. As we enter one room in West Park, Rookinella stops and tests the floor. It's made of wood, and it's sagging. "You have to know what you're doing, how buildings are constructed, and what happens to them when they're abandoned," she says. "You can't be foolish. Wooden floors get wet and people can fall through."
English Partnerships, the Government's national regeneration agency, is probably the one organisation that has to deal most regularly with urban explorers as most of the abandoned asylums and hospitals are now owned by them.
Mark Griffiths, head of estate management at English Partnerships, says the explorers are a persistent and worrying problem. "I wouldn't want to glorify any of the actions of these people," he says. "What they do is trespassing. They are unlawful and unwanted visitors. It doesn't matter what you call it, we can't condone an unlawful act that is highly dangerous and ultimately costs the taxpayer money."
The British exploring scene itself is largely divided into two groups; one which operates online, and "freelancers" who explore but don't publish the details of their exploits. The first group uses the website www.28dayslater.co.uk, named after the zombie horror film that featured a post-apocalyptic and abandoned London in its opening scenes. 28 Days Later operate openly, have a forum that anyone can view, emphasise using stealth, and tend to appreciate the historical significance of many of the sites they explore. The second group, called When Darkness Falls, communicate through a password-secured website only and, according to Rookinella, it is they who are responsible for much of the bad reputation that urban exploration enjoys.
"There's a lot of resentment against 28 Days Later from within the urban exploration community," she says. "Some see us as overly academic and technical, and I admit that we place a great emphasis on things like ropes and climbing, as well as appreciating the history of these places. There are other groups who just do 'mass invasions', where they storm a place and throw a party. That's their choice, but it probably helps to give urban exploration a bad name."
In recent months, a number of local newspapers have begun monitoring online exploring forums and have run stories on which buildings on their patches are being targeted. The explorers themselves are often heavily criticised, portrayed as dangerous youths and hooligans, something which Rookinella believes misrepresents the exploring community.
"I'm 22 years old, and I'm probably one of the youngest people I know doing this," she says, scrambling through another window that she says will take us into West Park's Great Hall. "The average explorer is normally in their late twenties or early thirties and holds down a perfectly respectable job. The motto of the community, and I admit it sounds a little corny, is 'Leave only footprints, take only pictures'. But generally, I try to avoid the whole ethics debate."
The roof of the Great Hall has gone and the walls are caked in black soot, the result of arsonists who nearly succeeded in burning the whole complex down shortly after it was closed. The hall, which once housed an elaborately painted stage, was the living heart of the asylum.
"Id love to have seen it before it burned down," Rookinella says. Asked whether she thought urban explorers were responsible, she looks offended. "No way," she says. "We would never destroy the buildings we explore. It was probably bored kids. Sure, there are times when you might need to break into a place and I'm not going to deny that people don't take the odd keepsake, but we have a huge amount of respect for these places."
The final must-see stop on our tour, I'm told, is the fully equipped padded cell that lies close to the children's ward, which is itself a rather eerie place thanks to the mottled, crumbling pictures of Disney characters that adorn the walls.
But at this point we realise we have outstayed our welcome. The asylum, which for the past two hours echoed to nothing more than the sounds of our voices, now has another presence – a rather angry-looking security guard brandishing a large wooden stick. Our attempts to conceal ourselves in an uncomfortable thicket of overgrown brambles prove fruitless and we are soon discovered.
The guard, dressed in a high-vis jacket and looking more than a little put out, is clearly used to dealing with explorers but knows there is little he can do other than kick us out. Explorers are well versed in how the law works and after a sustained but polite lesson from Rookinella on the ins and outs of trespassing, he decides not to bother the police and settles for ejecting us.
Walking back to the car, Rookinella can barely conceal her excitement at having successfully talked her way out of another scrape. "I won't deny that the risk element isn't a big part of it," she says. "There's nothing like a good security chase and successful escape, it's such a buzz."
Turning her key in the ignition, the satnav lights up – revealing three more "sights of interest" in the vicinity. "Where to now?" she asks. "There's another great asylum just up the road. And it has a mortuary."