Sheikh Abubakar holds a wad of notes to his lips and whispers an incantation, oblivious to the complaints of his latest patient. "I can't afford £500," says the sick man. "I have leukaemia. Here's £400. I need another £100 for food."
The sheikh, a medicine man dressed in African robes, stops chanting and puts down the money. But he is unmoved by his patient's plight. "Some people pay £1,000," he says softly, "some people pay £700. You will pay £500."
Seconds later, his £500 safely tucked away, the witch doctor fills a bottle with water, reaches into a compartment under his bed and pulls out an African rug on which is a cloth piled high with powder - medicine. He sprinkles three pinches into the bottle and shakes it vigorously before passing it to the leukaemia victim's right hand. It must not, he says, without explanation, be handled with the left.
"First, you rub the medicine here," says Abubakar, slapping his ribs. "The medicine will go straight into the blood. All problems inside will start to go. All problems with your body all finished."
Not only would it cure the patient's leukaemia, Abubakar promises, but, once drunk, it would also bring back a girlfriend who had recently left him. Magic! Black magic.
This scene wasn't played out in some poorly-educated village in West Africa. Neither is it an anachronistic, badly-scripted movie resurrected from the 1960s. It happened in east London this year and it is happening across the capital every day.
Inquiries by the Evening Standard have uncovered an exponential increase in the number of African medicine men practising in London. Many are recognised as traditional healers using plants, herbs and natural cures. But police and cultural experts say there is now a growing number carrying out animal sacrifices, charging extortionate rates to gullible poor people and, as we found, offering illegal and useless cures for cancer. Worse still, suspicion is growing that a trade in human body parts for medicine is either here or on its way.
In areas around Dalston in north London and Brixton in the south, some residents have reported receiving up to three flyers a week featuring witch doctors' claims to be able to lift black magic curses, cure disease, remedy sexual problems, bring riches and make court cases simply disappear.
The potion produced for our leukaemia "victim" - in reality Paul Davis, an actor hired by the Evening Standard - would cure no one's bad blood. Tests by the scientists who are also involved in the "torso in the Thames" murder have so far shown that it contains plant extracts, quartz and, disturbingly, blood and muscle tissue.
A team of forensic pathologists led by professor Robert Forrest of Sheffield University is trying to establish from which animal the blood came, but one thing is certain, according to Forrest: "It would have no therapeutic effect whatsoever."
Regardless of your beliefs and the desire not to ride roughshod over other cultures, it is difficult not to arrive at the conclusion that peddling bogus cancer cures to desperate people is simply wrong.
Since the discovery of a boy's headless and limbless body in the Thames in September 2001, the prevalence of black magic - the generic Western term for African voodoo and juju practices - has shocked detectives.
Until the murder of Adam, as the victim came to be known, there was a gaping hole in the police's knowledge of African medicinal practices - after all, it was not their job to pry into the religious or superstitious beliefs of other cultures. What they discovered during the Adam investigation changed all that.
It is believed that the boy was brought to London from Nigeria with the specific aim of being sacrificed to the spirit gods in order to bring luck to an illegal human trafficking operation. For legal reasons (the names of suspects have been passed to the Crown Prosecution Service) it is not possible to go into further details about his death, other than to say he was drugged, bled dry and had his head and limbs removed.
During the investigation that followed, detectives travelled to Africa and were shocked to find that the use of human body parts in traditional medicine was endemic. In South Africa alone, they found that more than 300 people are murdered each year to provide body parts for black magic cures.
During the Standard's inquiry, we were shown pictures of bodies plundered for organs, eyes or complete heads. In one, a woman lies on a slab, her body cut open from the chest downwards, her reproductive organs removed. In another, a child sits bolt upright in rigor mortis, his head removed and placed on an altar three feet above his ragged neck.
Dr Richard Hoskins, an expert in African and Caribbean cultures at King's College, acted as the cultural adviser in the Adam investigation. He has studied African healing for more than 20 years.
"For hundreds of years, sub-Saharan Africans have turned to traditional healers to cure their ailments," he says. "Even here in the UK, you will find that well-educated, professional people will go to such men to solve problems or cure diseases. The people involved in the Adam killing were very superstitious. There is an extreme fear and a belief in black magic through every stratum of African society. And rather than diminishing, I would say it is increasing in Britain."
It certainly is in London. Hundreds of witch doctors ply their trade across the capital, advertising on flyers or in the classified pages of the New Nation and The Voice newspapers. From Brixton and Peckham to Whitechapel, Stoke Newington and Barking - your local witch doctor is never far away.
We visited three natural healers and spoke to several others by phone (most are reluctant to meet indigenous white people). One, in Wood Green, claimed to be able to cure cancer. Another, calling himself a professor, in Dalston, demanded £550 for a cure for impotence. Another "sheikh" in London E15 confirmed that he could put a curse on a "business partner" who had supposedly ripped us off.
Our inquiries were hampered because Africans were too afraid to help. An intelligent, award-winning Nigerian journalist had to drop out of our investigation after visiting just one medicine man. "You shouldn't mess with these people," he said.
It is a problem that hampered the Adam inquiry. Belief and fear of black magic are rooted deep in the psyche of many Africans. "Much of what the traditional healers have learned over the centuries must not be underestimated - they have identified which plants are good for which diseases," says Dr Hoskins. "And Western medicine is now learning much from them. The problems arise with the use of human parts to cure ailments.
"In parts of Africa genitals and sexual organs are used to make medicine for sexual problems; eyes will be used to cure eye problems, blood will be used to treat blood disease, and so on. I once interviewed a muti [medicine] man in his home in Durban, South Africa. It was pretty scary. We were surrounded by jars full of human body parts. A good muti man will have a good contact at the local mortuary. But in many cases, people are killed for their body parts."
Dr Hoskins stresses that the overwhelming majority of believers in this country abhor what happened to Adam and simply reject the idea of killing to obtain a cure. But he is concerned that sociological forces mean it could happen here.
"The problem in London is that communities are dislocated. In small communities in Africa, a delinquent muti man could not get away with wrongdoing. Here, bad elements can slip in and out of areas anonymously, making unfounded claims and peddling spurious cures to desperate and gullible people.
"We know they are carrying out animal sacrifices, mostly chickens, rabbits and goats. My concern is that without the usual community checks and balances, it isn't a huge step for them to begin seeking human body parts."
Dr Yunes Teinaz, health adviser to the director general of the Islamic Cultural Centre and the London Central Mosque, says he is hearing increasing reports of attempts by unscrupulous practitioners to acquire human parts.
Sections of the Christian and Muslim communities in Africa practise voodoo in contravention of their religions. In Britain, the Muslim community is especially active in trying to stamp it out.
Dr Teinaz, who is also a practising environmental health officer, says: "There is a concern that human body parts are being used. We know that much of the bush meat trade is used in potions and ointments for black magic treatments and we know that other animals are sacrificed for voodoo purposes in the African community. But we have a deep concern over human body parts. We think they could be coming in with the bush meat."
The bush meat that concerns Dr Teinaz comes from animals illegally imported from Africa and used as medicine. According to black magic beliefs, certain animals possess special-properties. For example, the hornbill-bird is recognised as a dignified and graceful creature that can imbue calm in those who eat it. The black duiker deer, an animal with an inquisitive nature, is said to help the victims of theft find the thieves.
Whatever the perceptions of the good such "medicine" can do, Sheikh Abubakar was not prepared to share his secrets.
Photographer Mark Chilvers and I confronted him at his grimy flat on the Barking Road. He denied having supplied the potion, in spite of pictures and tape-recorded evidence to the contrary. The only potion he had, he claimed, came from a large container of vegetables soaking in a solution. Nowhere in sight was the special "medicine" he gave Paul Davis.
"I don't give medicine," he said. "I just provide luck - if your woman leave you or for examinations."
Detective chief inspector Will O'Reilly, one of the lead detectives in the Adam case, said police had had difficulty until now in successfully prosecuting bogus medicine men. "If the healer says he believes in his cure, and so does the patient, then it is difficult to prove fraud," he says.
"These people prey on the beliefs of desperate and gullible victims. However, we are concerned that even more sinister characters may be operating underground.
"In a shrinking world, people's practices travel with them. The Adam inquiry set us on a steep learning curve - to the heart of the previouslytaboo subject of ritual killings. We hope that the way we pursued our investigation will act as a deterrent to others."
We hope so, too. The police told me our potion was the first ever acquired for analysis in the UK. I told Abubakar it was medically useless and asked him if he knew it was a criminal offence under the 1939 Cancer Act - punishable by up to three months in prison - for non-medical people to prescribe remedies for cancer.
But, as if by magic, Sheikh Abubakar's English suddenly failed him.