The Priory, the favourite clinic of Britain's rich, famous and addicted, has just been valued at £875m. But is it really worth it? Terry Kirby analyses the secret of its success, while Virginia Ironside (below) describes her time on the inside

While others prefer vomiting therapy among the Buddhist monks in the Himalayas, having their blood changed in anonymous Swiss establishments or the charms of a detox at the Betty Ford Clinic in California, the Priory Hospital in south-west London has become the destination of choice for the modern British celebrity when grappling with the bottle, multiple drug abuse or just plain old-fashioned "stress" after too long on the showbiz party circuit.

Now the value of those celebrity customers - who bring the ordinary punters close behind them - has been made abundantly clear with the valuation of the company which owns the Roehampton establishment at more than £875m.

The Priory Group, which has more than 1,800 beds in 42 locations around the country, has been sold by the private equity firm Doughty Hanson to the banking group ABN Amro. In an indication of the booming market in private treatment for psychiatric problems, which include addictions to drink and drugs or eating disorders, Doughty Hanson is said to have made a return of at least four times its original equity stake, made when it first invested in the company three years ago.

ABN Amro is thought to have beaten off strong competition from a number of other venture capital groups interested in the company. As one person involved in the sale process said: "I don't think the valuation would have been so high and people so keen to buy if they didn't believe that that profits were just going to get bigger and bigger.''

Those profits are, at least in part, based on the fact that, for several years now, the Priory has seemed an almost inevitable destination for many, once all the right buttons of celebrity have been pressed.

Take Kerry Katona. This is a young women possessed of suitably enhanced and photogenic looks, formerly an Atomic Kitten, winner of a celebrity reality series, married and then separated from a former boy band member, who has endured a particularly torrid time at the hands of some sections of the media, and who, lo and behold, suddenly finds herself at Roehampton and "in rehab", being treated for an addiction to anti-depressants.

It's about as predictable as seeing her on the pages of Heat magazine, falling out of the Met Bar or Chinawhite. Julian Linley, the magazine's deputy editor, believes celebrities like Katona pick the Priory largely because it's an easy option.

"It's a bit like eating at the Ivy," he says. "There are lots of other great restaurants to go to, but people chose it because it's simply the familiar and easy place to go. They know the Priory because its got a name you can remember and it's close to central London so all your friends and family can come to visit you. And if people are having problems, it's where their friends tell them to go, because it's the first place to come to mind.''

Indeed, the list of former patients is long - Moss went there for treatment after splitting from Johnny Depp, while her current boyfriend Pete Docherty has received treatment for his heroin addiction. Comedians Lenny Henry, Caroline Aherne and John Thompson have had treatment.

Others who have been helped there include the footballer Paul Merson and several members of the rock aristocracy, including Eric Clapton (cocaine) and Ronnie Wood (alcohol). The "It" girl Lady Isabella Hervey was treated for bulimia, the television presenter Gail Porter and the stepdaughter of actor Pierce Brosnan all checked in.

For a basic rate of around £650 a day, the modern British celebrity will get access to some of the best specialists in treating addictions and stress problems in a plush, hotel-like environment, where many of their whims - apart from those relating to the finest wines or lines, of course - are catered for.

While the media likes to stress the luxurious nature of the Priory, the reality is not quite the same. "I think a lot of celebrities like it because it actually feels more like a hospital than a health farm. There is a sense they are getting genuine treatment. And sometimes in the company of Natioanl Health Service patients,'' said Mr Linley.

One former senior doctor at Roehampton, who asked not to be named, concurs. "It's definitely not a health farm and I would say that the ambience is more three star than five star. I think the place is a bit second rate, frankly: some of the carpets are worn and the paint is peeling. And the staff there are like Harley Street - some of the very best people in their fields are working there, and some of the very worst.''

Another issue, he said, was the recidivism rate of patients. "Many GPs in west London are angry with the Priory because they say it doesn't get results.''

He added: "I think most people would actually get as good a deal going to one of the Betty Ford or other clinics in the United States - like Naomi Campbell has - rather than be under the spotlight at the Priory. But people tend to go to the Priory as a result of word of mouth recommendations for the most popular doctors and consultants.''

Although most celebrity patients at Roehampton will fund themselves, the majority being treated there and at the group's other hospitals will have some kind of private health insurance, usually related to their professional capacity.

Recently the group has launched advertising campaigns in national newspapers to target high-flying City workers suffering from alcohol problems - most of whom will have some kind of private health insurance. The Ministry of Defence last year signed a three-year, £5m contract with the group to treat service personnel.

While celebrities and City executives are the most high-profile element of the Priory's business, the group makes the bulk of its money in other ways. It has expanded the bread-and-butter side of its operations and likes to stress its work with young people suffering from conditions such as autism and Asperger's Syndrome, along with its clinics dealing with neurological rehabilitation. The bulk of these patients - as well as a small number at Roehampton - are NHS referrals. It is believed that around half the group's total business comes from patients passed from the state sector.

William Lang, of the healthcare analysts Lang and Buisson, said: "Although it attracts attention, the celebrity side of the business is only a small part of the group. There are quite a few areas where they are the only serious presence and they receive a substantial number of NHS referrals because there is no state provision, such as the special schools for treating children with disorders and the clinics dealing with rehabilitation for people suffering from brain injuries. They are filling a gap for types of treatment that the NHS cannot provide.''

Yesterday's sale of the group is likely to swell the bank balance the group's chief executive, Dr Chai Patel, a New Labour donor who, along with members of the senior management team, owns around 14 per cent of the group. Dr Patel, then head of Westminster Health Care, bought the group in 2000 from an American concern which had owned it for more than 20 years. Since then, the group has added 11 sites. It now now includes 15 psychiatric hospitals, seven schools, two therapeutic community assessment services and five brain injury and rehabilitation units. Many of its hospitals are housed in fine listed buildings.

Although the Priory Hospital has only recently come into public notice thanks to the nation's obsession with celebrity, it is, in fact, London's oldest private psychiatric hospital. It has been in continuous operation since 1872, when Dr William Wood, one of the first modern psychiatrists, moved his patients from Kensington to Roehampton to benefit from the calm atmosphere and clean air of the countryside.

On its website the group states that its overall vision is: "To bring hope, healing and sanctuary to all the patients, pupils and residents who need our services." Even if the worst thing to happen to them is having taken part in a celebrity reality show.

'There's no better place to be when you feel that life isn't worth living any more'

It's a few years since I was admitted to the Priory. I had two bad bouts of depression and found myself a guest at the hospital in Roehampton, a huge, white castellated building, set in a wide lawn. I was in once for a month and once for a week - and I can't think of a better place to be when one feels that life isn't worth living any more.

At the Priory Hospital, I found asylum. I could give in completely, and just rest, cry and sleep. Even in the middle of the night there was always a nurse available to come to my simply furnished, but elegant bedroom (all have tellies) and hold my hand and say that I would, she was certain, get better. I was given proper treatment and medication, and each day had some kind of structure, even if it was just meals in the communal dining room.

At the Priory you not only have your psychiatrist who visits you every day to see how the medication is working, but also a whole range of other activities, once you start to feel better, from drama therapy, flower arranging, cognitive therapy, swimming, yoga, walking... in many ways it is like being in a luxurious health farm, the only difference being that everyone in it is deeply disturbed or unhappy.

There's a huge Gothic chapel with a piano; or you can play Scrabble or do one of the large jigsaw puzzles left lying around.

The presence of other people was healing, too, people who were just as desperate and unhappy as I was, and some even worse. There were people who heard voices, people who stood on their heads and muttered, people who had tried to kill themselves several times, anorexics who seemed on the point of death. I didn't see a single glamorous person there - and half the patients, anyway, are referred by the National Health Service.

Because of its number of star guests, the Priory has become something of a joke. People imagine that if you are rolling in money, and suddenly feel a bit blue for a couple of days, you sail off to the Priory in the same way that you might go to a health farm - for a bit of pampering. This is a complete fiction. The truth is that everyone there is suffering. But if you have to suffer, I'd prefer to suffer at the Priory than anywhere else.