TALES of black magic are not common fare in British courtrooms. But for the past two months, jurors at Birmingham Crown Court have learned the grisly rituals of a juju ceremony. They heard how a British nurse, Josephine Iyamu, forced five women to eat chicken hearts and drink a cocktail of blood and worms. Once the women swore their allegiance, she arranged for them to travel to Europe, at a cost of €30,000 ($35,000) or more. They were forced to work as prostitutes to repay the debt to the woman they knew as “Madam Sandra”. The jury found her guilty of slavery offences and perverting the course of justice. On July 4th a judge sentenced her to 14 years in prison.

Yet no part of the crime took place in Britain. The women are from Nigeria and were forced to work in Germany. It was the first time a court had used Britain’s Modern Slavery Act, introduced in 2015, to convict someone of an extraterritorial offence.

Most British legislation applies only to crimes committed within the country. But a few laws, such as those against murder and the sexual abuse of children, can be enforced regardless of where they are broken. Parliament added modern slavery to this list to prevent British criminals escaping prosecution in countries with less stringent anti-trafficking laws, and because the crime is often spread across borders.

Britain’s campaign against slavery is hardly new. In the 19th century the Royal Navy sent a squadron to the Atlantic to break up the trade, seizing slave ships and freeing their human cargo. Politicians who support the new law proudly invoke this history. But the definition of modern slavery is much broader than its historical equivalent. A Home Office briefing note lists 17 types of abuse, ranging from forced labour to coercing spouses or servants into domestic work. Criminals who force others to commit crimes, such as kingpins who send teenagers to deal drugs, also count towards the tally.

Existing laws banned most of these practices, but the new act introduced stiffer penalties, including lifetime jail sentences. Police recorded more than 5,000 possible victims of modern slavery last year; the Home Office estimates that in 2013 the true figure could have been 10,000-13,000. Theresa May, the prime minister, championed the legislation as home secretary and has called modern slavery “the great human-rights issue of our time”.

Yet Ms Iyamu’s conviction is the exception, not the rule. Only 6% of the modern-slavery offences recorded by police in the year to March 2017 resulted in charges or court summonses. Officers point out that charges are sometimes brought under different laws, meaning the total proportion will be higher. Even so, a report in 2017 by the police inspectorate raised “serious concerns” about the quality of modern-slavery investigations, and found some officers did not fully understand their new powers.

Even the most committed cops can struggle to build a case. David Lewis of Dorset Police says victims sometimes don’t understand that they have been exploited if they are paid more than they would have earned in their home country, making the case hard to prosecute. Ms Iyamu’s victims had to overcome their fear of the juju oath, as well as learn to trust the police, says Kay Mellor of the National Crime Agency.

The law also requires businesses with a turnover greater than £36m ($48m) to publish a plan to prevent abuses in their supply chain. But estimates suggest only 30% have done so. A parliamentary committee recently concluded that the government’s “hands-off” approach “is not working”. Kevin Hyland, the anti-slavery commissioner, says the Home Office should keep a register of all companies required to have plans, and fine those which fail to comply. If Mrs May wants this flagship policy to succeed, she will have to pair her strong words with actions.