If you are shocked by the proposition that there are slaves in modern Britain, think again. Today's "slaves" are not out in the open, like the Africans transported to the US in the 18th and 19th centuries, and except in the most extreme cases they do not wear chains.
They live in unremarkable houses and flats where they are forced to do hard physical labour, provide "services" or, in some cases, both. That police rescued three women from alleged domestic servitude in south London is startling because of the time-scale, which apparently covers three decades.
Not much is known about this extraordinary case but it has focused attention on a problem which is mostly hidden from view.
Concealment is the key to modern slavery. According to the International Labour Organisation, almost 21 million people are victims worldwide, with women and girls outnumbering men.
Trafficking for prostitution and exploitation has received a great deal of attention but forced labour is actually more common, especially in the domestic and agricultural sectors. I have interviewed victims of both and in each case they were tricked by trusted family members or friends.
Last year I met a 41-year-old woman from the Caribbean who spent almost a year in domestic servitude with a middle-class Nigerian family.
"Crystal" agreed to talk to me for my book The Public Woman because she wanted people to understand what is going on behind closed doors in this country.
"These people could have killed me and no one would have been the wiser," Crystal told me. Like most victims of human trafficking, she knew she was in the UK illegally and was too scared to go to the authorities in case she was deported; she had suffered extreme domestic violence at home and was terrified of her ex-husband, who had threatened to kill her.
Traffickers encourage their victims to believe that the police won't help them, and they use tried and tested methods to break down their self-esteem. In Crystal's case, the Nigerian woman who paid for her to come to England knew she had been beaten and raped by her husband. They were both evangelical Christians and spoke on the phone many times before Crystal got on a plane; it was a classic grooming process but the woman ruthlessly exploited Crystal's vulnerabilities once she was in her power. "You are like filth", she told her.
"The only thing you are good for is cleaning the kitchen and my baby's bum."
I was reminded of a harrowing interview I did with a Ukrainian woman in her early twenties. She had been sold by her father and uncle, without her knowledge, to a gang who brought her to London and sold her on to Russian gangsters. "They said I'm their property, I will be with them for the rest of my life – I'm not human, just something that can be bought."
It is this daily experience of intimidation and abuse – what a senior police officer calls "invisible handcuffs" — which prevents so many victims contacting the authorities. It is a chilling fact that some of the most vulnerable people in this country do not know they have any rights, and their exploiters go to great lengths to stop them finding out.