The danger of their work and the tragedy behind the trade was revealed recently when the pair travelled with their team and 12 parents to a town in northern India to rescue a group of girls trapped in the notoriously abusive New Raj Kamal Circus.

The information they had received indicated that the children were being subjected to regular sexual abuse as well as being forced into dangerous work in harsh conditions. They were also told the circus owner had links to local gangsters and the police.

'There is a danger of being killed,' Mr Shrestha said casually.

'On the one hand, we are worried,' Mr Lama said.

'But when you chase the traffickers and catch them it's very pleasurable,' his partner concluded.

By drawing on their intimate knowledge of this murky world, the two have become an effective double act.

According to Lieutenant Colonel Philip Holmes, a retired British army officer who founded the charity in memory of his first wife, there are about 500 Nepalese children, mostly girls, in slavery in Indian circuses. Nepalese are preferred for their pale skin. Colonel Holmes said there were several reasons parents agreed to sell their daughters to the travelling circuses, typically for about 6,000 rupees.

'Probably the main factor is poverty, a real grinding poverty in the village areas,' he said. 'It has to be said there is also an element of greed, and there have been some pretty nasty parents out there who are very quick to off-load children from a previous marriage.'

Although the Trust says that trafficking to circuses is limited to cases numbered in the hundreds, it is part of a larger phenomenon, with the brothels of Indian cities such as Mumbai and Calcutta holding thousands of trafficked Nepalese women and girls. According to staff at the Trust, many circus children are at risk of ending up there once their circus careers are over.

Some measures have been taken against trafficking from Nepal, including publicity campaigns and closer scrutiny of travellers at the frontier. But in a poor and chaotic country with an open border to India the problem persists.

Parents of the children are easily deceived. One of the fathers on the rescue mission was Gopal Bisworkarma, 34, who had repeatedly tried to retrieve his daughters, aged 10 and 13, but was driven away by the circus owner.

'I hadn't realised, but it is written in the contract that if the girls are released early we have to give him 1,000 rupees per month each for the duration of the contract,' said Mr Bisworkarma, a blacksmith struggling to feed his other three children on an income that amounted to just a few cents a day. 'The contract is for seven years and they made us mark it with a thumb print. Had I been able to read I would never have done it.'

Finding the New Raj Kamal Circus was not easy. The team began by visiting a tent maker and asking discreet questions. Then they called on a trafficker, who asked them to supply more girls. Finally they visited a big wheel at a fun fair that belonged to the same man who owned the circus.

Assembling all the clues, they concluded that the circus was hundreds of kilometres away in Allahabad, in the state of Uttar Pradesh. The rescue team took a rickshaw to the nearest station.

But the team's cover had been blown, and the information about Allahabad was a trick. After a week of searching and many nights on trains, slowly criss-crossing the state, they eventually found the circus close to where they had started their hunt, in Kushinagar.

On arrival, the rescuers found a heavily patched big top surrounded by a corrugated tin fence painted with crude images of performing girls. A small, bedraggled crowd was watching one of the three daily performances in silence. The manager was drinking tea and eating sweets with the local police.

The police took the desperate parents inside the tent, but locked out the rescuers. As they waited outside, activists told the local police chief that he was legally obliged to refer the case to the government's Child Welfare Council (CWC), but he made it clear he was taking the law into his own hands. 'Here, the CWC won't decide anything,' he said. 'Here, I'm the one who will decide.'

Inside the big top, circus officials tried to persuade the children to stay, claiming that the circus would be forced to close without them. But the girls insisted and, hours later, eight of them emerged.

Some of the parents were distraught when they found that their children had been hidden.

That night, the rescued girls told us that they had all been abused by the circus owner.

'I was sleeping when he came,' said one 15-year-old. 'He hit me on the back. It was painful. He hit me many more times. He said that he would kill me, and then he raped me. If anyone came to know, he said that he would kill me. So I did not tell any of the girls. I did not even tell my best friend.'

The girl recalled how she and her friends had secretly planned to kill their attacker. 'We used to say, 'If he comes to us again, then we will kill him and go to the police and tell the police everything, even if they lock us up.' That's what we used to say.' 'He didn't let us sleep at night,' a 13-year-old told us. 'He really troubled us. And he would fondle us all over our bodies. We never slept. We had to stay up all night long. I hated getting fondled during the night.'

Some of the girls had brutal marks where bones broken in trapeze accidents had not been set properly. One of them described her accident. 'To do my act, I had to hook my foot onto a rope. My foot was peeling so I said I didn't want to perform. I asked the manager to let me off for one show, but he said that I had to work. I was crying.

'I do not know how I fell that day. I don't know how my foot came loose - but I landed on my arm and it broke.' After that, she said, whenever the owner wanted to frighten her he would threaten to remove the metal pins that had been used to set her arm.

The girls also spoke about their fantasies of escape. 'We used to say that all of us would get together, take all the children, make a tent and escape to Kerala [in southern India],' one of them said. 'We vowed that we would not leave anybody behind.'

The next day, the rescue mission went back to the police station and four more girls were released. But even as they were handed over, a mob of several hundred thugs gathered to intimidate the rescue team. Some of them carried wooden staves. As the police stood idly about, the circus manager threatened to kill members of the rescue mission.

Finally, the team succeeded in rescuing 20 girls from the circus, the youngest just 10 years old. One girl is still missing, but the Kushinagar police have refused to file a case against the circus owner, even though they have evidence of child abuse and slavery.

The girls who were freed are now receiving education and care at shelters in Nepal run by the Esther Benjamins Trust.

Some of them are learning to make mosaics for a living; others are in schools.

'We used to be concerned that if we gained weight, we wouldn't be able to do the tricks,' said 15-year-old Sunita Lama. 'Now that we don't have to worry about that, we eat a lot.'

After the rescue team left the police station for the last time, one of the team received an anonymous call on his mobile phone telling him that the driver of the final vehicle in the convoy had been kidnapped by police. The caller said the jeep was being chased.

Alarmed, the rescuers urgently telephoned the Uttar Pradesh state police headquarters. It was a Friday evening. 'Call back on Monday,' they were told. From the speeding vehicle the team protested that urgent action was needed. 'OK,' the police said. 'Send a fax.'

Luckily, the convoy made it to the next town before being caught. The terrified passengers of the last car finally scraped in later that night. The driver had been held by thugs and beaten for about half an hour before being released.