Johann Hari: This murder illuminates a darker truth

If you are already here and prepared to pay taxes you should be given legal rights

This is the story of an everyday psychopathic murder – but if we follow the trail of blood, it leads us to 200,000 terrorised Chinese people living in Britain's shadows, and to a secret we do not want to see.

You walk past women like Xiao Mei Guo all the time. She was a 29-year-old Chinese woman who stood outside my Tube station in the East End of London and tried to sell pirated DVDs.

For her, our grey skyline was the end-point of an epic four-month journey across mountains and borders, on foot and in lorries, so that her two young sons back home would never have to live like this.

One day in August last year, a man called Derek Brown approached Xiao and offered to buy a large batch of DVDs if she agreed to come to his flat to prove they worked. She followed him. Nobody ever saw her again. He had bragged to a friend that he was going to become a "famous" serial killer, and so he came here, to Jack the Ripper's old stomping ground.

The court – which reached a guilty verdict this week – was told that Brown picked out Xiao because "nobody would miss her". She was an "illegal". She had no rights. She was part of the moneyless shadow-world that swirls around the rich world, unpoliced and unprotected. And Brown was – in a terrible sense – right. Xiao was a rare exception: many "illegals" die with nobody paying a price.

Xiao's husband said this week: "The children ask me 'Where is mum? We haven't spoken to her for a long time? We miss her. Can we talk to her?"

Thanks to an extraordinary piece of investigative journalism, we can now begin to patch together the story of women like Xiao.

The British-Chinese writer Hsiao-Hung Pai decided to go undercover among Britain's 200,000 undocumented Chinese workers for her book Chinese Whispers. She found that many people do not survive the journey here. Some are abandoned on freezing mountaintops crossing into Europe; others suffocate in lorries.

Ask your friends: what was the biggest act of mass killing in Britain since the war? It was not 7/7 or Dunblaine or Omagh. It was death of 58 Chinese men and women in a lorry in Dover in 2000. Even though the temperature outside was 30 degrees, the truck driver who agreed to transport human beings decided to switch the refrigeration off to save fuel. The people inside howled and screamed and banged – and slowly cooked. But Xiao got here – and promptly got picked up by the criminal gangs who feed on illegal immigrants.

At first glance, DVD-selling looks like an easy job for a beginner. You buy the DVD for £1.75 a disk and sell it on for £3. But it isn't that simple. The trade is controlled across Britain by armed criminal gangs. Once you start selling for them, you are given strict targets to sell more and more DVDs each day. And if you try to get away?

Pai tells the story of an ordinary Chinese man called Ah-Hua. He started selling 10 DVDs a day for a notorious gang known as the Dong Bi, which operates all over Britain.

But soon they were making impossible demands on him. When he couldn't take the pressure any more, Ah-Hua told them he wanted out. Pai explains: "That night, five Dong Bi gang members pushed their way into his flat. They beat him up until he lay helpless on the floor. Then they dragged him out of flat, blindfolded him, and drove off into the night."

He was held captive for three weeks. His captors told him that they had located his family in Fuqing by going through his mobile phone. They would release him if he could pressure his family to come up with £12,000 to "buy" his freedom. Within a week, his wife had borrowed the money at sky-high rates of interest. Today, he works in Chinatown as a kitchen porter, paying off the price of his freedom. Was this Xiao's story? Was she seized by one vicious thug from a whole gang of them? Pai certainly found that the violent intimidation of Chinese illegal workers was routine in Britain.

When she asked the most basic questions of her gangmasters – like why people were being paid just £1 an hour – she was told: "I can make anyone disappear, just like that. You can be thrown into a river and your parents in China would never find out what happened."

The language of "illegals" distances us from the fact that people like Xiao are just like us. If not for the lottery of birth, they would be us.

When the 23 cockle-pickers on Morecambe Bay realised they were going to drown, they used their mobile phones to call home and say what you would say: "I love you".

How do we stem this abuse? How do we free the Xiaos who remain on our streets?

After Morecambe Bay, the Government's actions focused on licensing and monitoring the middle-men – "gang-masters" – who provide labour for our farms, food processors and shellfish companies. That's valuable, but by definition, only people here legally can be regulated.

What about the half-a-million people who are here illegally and whose gangmasters can do anything they like to them, knowing they can never go to the police?

It is inconceivable that we will deport all those people even if we wanted to; no democracy in peacetime has ever shipped out so many. A few brave politicians – from Nick Clegg to (surprisingly) Boris Johnson – have supported the only realistic solution: an amnesty.

If you are already here and you are prepared to pay taxes and perhaps a fine you can pay over time, you should be given legal rights – and encouraged to testify against these gangs.

The extra £1bn in taxes we will raise can be earmarked for more border police. It is only by making it possible for their victims to come forward that we can begin to break the Triads and snakeheads.

We are too late for Xiao. We are too late for Ah-Hua. We are too late for Morecambe Bay. We are too late for Dover. We are too late for hundreds, terrorised in the shadows, whose names we will never hear. How many more people do we want to be too late for?