If you were to invent the darkest possible television detective drama you might come up with a tough cop, a counter-terrorism expert battle-hardened in the military police and Northern Ireland; turned, now he has a family, to the hardest job of his life – saving children from paedophiles.

You might give him a name like Jim Gamble (“he only needs to be lucky once”) and you might even stage the last scene at an airport, where Gamble, on his way home, hears that a child his team has spent months searching for has been rescued.

Except that all this is terribly real. Mr Gamble, sitting in his gloomy London office of the Child Exploitation and Online Protection Centre (CEOP), where he leads a 100-strong squad of police, intelligence officers, psychologists and technical experts, is real. And sadly, so is this child, who was rescued recently after his team painstakingly pieced together clues from images circulated by paedophiles.

“You suffer the trauma of living with that child’s abuse as you follow the still photographs, the video photographs and see them progress in age. Then every now and again you will get those pieces of evidence, and you get a phone call from the team to say such and such has been recovered. That is the point where the emotion is so overwhelming, the only thing I can compare it to, genuinely, is the birth of a child,” Mr Gamble said.

“The day that I learnt that we had found that child, I was in Gatwick airport. I could have flown home without a plane. The first thing I did when I got in was to hug my youngest daughter, because there’s something in that which just makes you feel this is right, this is safe.”

It is the prospect of saving a child that sustains him in what is the grimmest of jobs. His team races the clock to find an abuse victim – and jail the predators – all the while seeing the images of suffering traded online. Child abuse is a growing problem in Britain, fuelled, he said, by the internet. “You’d think coming from a background where you’ve been involved in a counter-terrorist campaign, where you’ve been involved in serious and organised crime, you’ve pretty much seen it all and nothing will shock you,” he said softly. “Nothing that I did in my previous work would have prepared me adequately for the work that we do now, the trauma involved.”

On this particular case, his team “was searching for a young child for many, many, months”. Officers gauge how recent a picture or film is, look for clues in the background or on the bodies of the child or its abuser, and follow the digital trail left by cameras and computers. Each image “we treat like a crime scene”.

Often they go undercover online and strike up a relationship with the abuser. This week, when CEOP helped to catch the ringleader of one of the world’s biggest online paedophile rings, Timothy Cox, they took over his internet operation for ten days, enabling them to trace as many of the paedophiles using it as possible.

“I’ve seen lots of photographs of actual physical abuse that you couldn’t comprehend and you wouldn’t want to write down. It leaves you aghast at how someone could be that depraved when dealing with another human being, let alone a child. To the degree that you do begin to question humanity,” he said.

But “most shocking of all”, he said, was witnessing the mental torture in a child they had got, virtually, to know.

“When you have access to images along the progression of the abuse against that child, almost more horrifying is that which makes you look beyond the physical hurt, to the mental anguish and trauma, that sees a young boy or girl speaking as if they were an adult, relating to sex in an environment that you wouldn’t imagine was even within the UK.”

Inevitably the distress of the job is magnified as, over time, they start to care deeply about the fate of the child, but see the abuse intensify. “We give everybody a name. You say this child, I think their name might be this, we think they may be in this area and they get some more pictures of the abuse. And some more.

“You begin to form a vicarious relationship where you are so desperate to find that child, so desperate to stop the harm that they are feeling, you become massively frustrated when you can’t get any more clues.”

Sometimes, just when they think they have located a child, “suddenly the whole thing falls apart and you are in the wrong part of the country at the wrong place. It’s very hard to lift yourself after that,” he said. “We’re talking about the most emotive area of most people’s lives. And that’s about our love for our children. As a father – I have three children – there’s nothing that’s more important to me in the world.”

This is why the job is “personally and professionally corrosive”. We wondered how he and his staff coped, looking through sickening images hour after hour, day after day, sometimes witnessing live abuse online, reluctantly inhabiting the world of people whose ambition it is to “rise up the hierarchy” of a paedophile ring.

Mr Gamble talked, remembering his counter-terrorism days, of the “threat” this work poses to staff. “People will work harder, they’ll work longer. They will stretch themselves. The people that work here all work here by choice. You have to throw some of them out at the end of the day.

“You’re so heavily involved, you become used to the level of threat. It’s only when you leave that, for a period of time, and walk back into it that you recognise the trauma of the environment that you operate within.”

They have psychologists on hand, they are a close-knit team, and photos of happy children are displayed on the walls of the offices. “It sounds corny, but people can look up and it gives them some relief,” he said.

Since the pioneering CEOP was established last year it has had great success. In this week’s Timothy Cox case, 31 children, some as young as 18 months, were taken into care, and several hundred paedophiles were captured. That aside, CEOP has rescued 76 other children in Britain in its year of operation, and arrested 83 high-risk sex offenders. On its website pictures are posted of the most wanted offenders who have gone missing. Four of the first five were captured within four weeks.

It makes you wonder whether the crimes that Mr Gamble and his unit have uncovered are just the tip of the iceberg – whether, if they had more resources, they would catch ever more abusers. Does he believe that the danger to children has increased because of the internet? “That goes without saying. I absolutely accept that.”

Digital photography, too, has made it easier to take images and post them online. Networking websites popular with children make ideal hunting grounds for predators. But, Mr Gamble said, as his unit’s technological expertise was overtaking that of the paedophiles: “In the longer run we are going to make children safer, as we develop a much stronger understanding of the environment, as we are more aggressive in our pursuit.”

There is still much of the soldier about Mr Gamble. So he was furious when remarks that he made last week were interpreted to mean that he had “gone soft”, and thought that some paedophiles should not go to jail. Except for the very “grey areas” – a 19-year-old looking at a picture of a 15 -year-old, say, and for whom a severe warning may be all that is necessary – he is absolutely clear: he would jail the lot of them if he could, build a new prison if necessary. He regards them as evil.

“There is no halfway house. I hate them.”

He insists that they need far tougher surveillance on release from prison. Along with chemical castration and tagging, he is pushing for the Government to adopt a technique familiar from his counter-terrorism days: undercover surveillance. “Being able to survey them in the same way as we do terrorists, in the same way as we do organised criminals, ascertain their pattern of life, means that we are able to make judgments that protect our children much more effectively.”

He also said that Britain needed to improve the support given to the victims; for them their rescue is far from a happy ending, but at least “it’s a beginning of a new phase where that child isn’t going to get up tomorrow morning waiting in fear for the door handle to turn and the predator to come in”.

So, we returned, with the full force of Mr Gamble’s determination, to the children saved. All the staff were emotionally involved in the cases, he said: “When a child is rescued, it goes round this building like wildfire. And no matter how hard that day has been, no matter how difficult and corrosive the other work we do, people walk on air for that day.

“And that is what makes this job worthwhile. While I’ve said it’s frustrating, that’s what makes it the best job in the world.”

One day last year Mr Gamble knew that he had found his life’s cause. He was on a plane coming back from Cambodia, where he had met little boys and girls living in rubbish dumps, being abused by European tourists for £4 a night. A book he was reading contained a poem:

“I spent an hour with laughter, she chatted all the way.

But I barely remember a single thing from what she had to say.

I spent an hour with sorrow, and n’er a word said she.

But oh the things I learned the day that sorrow walked with me.”

And this is what he does: spend time with sorrow, with the suffering of children, to put a stop to it.

Jim Gamble

Age 47

Career Military police: West Berlin, Belfast bomb squad; RUC 1982-2002, head of Special Branch in Belfast; deputy director general of the National Crime Squad, 2002-06, responsible for serious and organised crime. Chief executive of the Child Exploitation and Online Protection Centre, April 2006

Lives Belfast, three children

Hobbies Black belt in jujitsu; spending time with family

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