EMMA ALBERICI, PRESENTER: On the 5th of November 1979 the son, of the chauffeur at the Australian High Commission in London went missing.

The disappearance of Martin Allen sparked a massive manhunt but he was never found. Did he run away or was he abducted?

As each decade has passed, small strands of evidence have appeared, many of them suggesting that Martin Allen might have been taken by a paedophile ring with links to people high up in the British establishment - including MPs at Westminster.

Several people trying to investigate the allegations have now been told to stop asking questions. Reporter Mary Gearin tracked them down, trying to put together the pieces of this most baffling case.

MARY GEARIN, REPORTER: Great power potentially bringing great abuse.

JOHN MANN, LABOUR MP: Repeatedly, police investigations into child abuse across Britain were being stopped by people in senior positions. And it's one of the critical questions we want answers to: Who stopped it and why?

MARY GEARIN: At the centre of this scandal are accounts of paedophile parties for prominent establishment figures, abusing children, mostly boys.

Many of the claims are linked to this place: Dolphin Square, an apartment block near the Houses of Parliament; while MP Cyril Smith, who died in 2010, has been named as one of several politicians to have abused boys at another location, Elm Guest House in South London.

CLIVE DRISCOLL, FMR SCOTLAND YARD DETECTIVE: You know, it was a group of people that obviously had a particular sexual appetite which is unacceptable and disgraceful.

MARY GEARIN: The brother of one missing boy is sure of a connection to the sickening events.

KEVIN ALLEN, MARTIN ALLEN'S BROTHER: He was probably mistreated and abused for maybe a couple of months and thrown away, just like a ragdoll.

MARY GEARIN: Kevin Allen still misses his little brother Martin, once Britain's most famous missing boy.

The family once lived on the grounds of Australia's High Commission in London. Their father Thomas was chief chauffeur for more than a decade.

Family happy snaps show their bird's-eye view of London's hoi polloi. But everything changed for the Allens when 15-year-old Martin went missing on November 5, 1979 as he took the Tube to another brother's place.

KEVIN ALLEN: First of all, he was treated like a missing kid. And then, sort of word got out that there was the embassy link and then I think that sort of oiled the wheels a bit.

MARY GEARIN: The only clue was one witness report of a man with his hand on a nervous-looking boy on the Tube. It came to nothing. Heartbreaking days turned into weeks and months. Then 17-year-old Kevin Allen received a jolt when he floated one theory past a visiting policeman.

KEVIN ALLEN: And I just said that higher people must have taken him. Someone's sorted him away. And the detective said to me, "If you keep saying things like that, you could get hurt."

MARY GEARIN: And you believe that this was a detective shutting down something because he knew something?

KEVIN ALLEN: I believe so. Yes, definitely.

MARY GEARIN: Last year an anonymous man known as Nick came forward to claim, from the age of seven to 16, he was driven to Dolphin Square to be horrendously abused. He also claims to have witnessed three boys murdered: one by a Tory MP, another murdered in front of a former cabinet minister.

Scotland Yard has called the claims "credible and true." Police won't confirm if Martin Allen is one of the murdered boys but officers involved in that investigation have interviewed Kevin.

(To Kevin) And you're sure that Martin's case is linked to this so-called VIP abuse ring?

KEVIN ALLEN: Yes, definitely. Definitely.

JOHN MANN: I have little doubt that this is connected in. Exactly who, how, when, where: I don't know.

MARY GEARIN: Labour MP John Mann has been campaigning on this subject for years. His own investigations were stymied in 1989 when, as a councillor in Lambeth - just over the river from Westminster - he gave police evidence pointing to the abuse of children from local care homes in the area, at least one of whom now says he was taken to Dolphin Square.

JOHN MANN: And for three months they investigated. And then the investigating officers told me they'd been instructed by people on high in the police to stop all investigations, despite the fact we had a lot of documents and a lot of evidence. And that was that.

MARY GEARIN: Mann has now passed to police a dossier naming 22 politicians accused of involvement in the paedophile ring.

JOHN MANN: There are people still involved in public life who ought not to be and who ought to be in front of the courts.

CLIVE DRISCOLL: I think Lambeth was something like the fifth or sixth largest police service.

MARY GEARIN: Clive Driscoll has also faced high-level resistance to his uncomfortable questions. The retired Scotland Yard detective has had a long and distinguished career that almost went off-track in 1998, soon after he started probing into alleged abuse at Lambeth children's homes involving prominent figures.

CLIVE DRISCOLL: So within two weeks of that investigation starting that I was asked to attend the Barnes police station whereupon it was suggested to me that I'd mentioned certain names as suspects - which I had: 100 per cent had said them - and as a result of that, they removed me from post immediately.

MARY GEARIN: An old-fashioned officer, he still can't say what happened to him constituted a cover-up. But he still wonders what might have been.

CLIVE DRISCOLL: The person who actually interviewed me he said, "Look, I'm sorry. I feel quite shabby about this." So it wasn't everybody. But I just... At the time, it didn't actually make a lot of sense.

MARY GEARIN: Driscoll had received information from Liverpool, the precinct that reportedly uncovered a shrine to Martin Allen at a local paedophile's home. But he can't say he's unpursued evidence would have shed light on that case.

The former policeman is optimistic by nature about the system but he says, even now, many officers won't speak out.

CLIVE DRISCOLL: No, there is fear. I mean, I've had police officers contact me personally and I think they would dearly love to come forward but there is fear. There is immense fear.

MARY GEARIN: The only answer is to find more people willing to tell their story.

JOHN MANN: There'll be people in Australia today who left Britain because of the abuse they had as children in order to start a new life and getting away from it. And what we need is any bit of information. Any bit of evidence, whether they decide to go public, whether they decide to prosecute or not, can be vital.

MARY GEARIN: Kevin Allen has been left deeply cynical. His father, in particular, suffered.

KEVIN ALLEN: He used to go out with the dog and... and shed his tears, you know, and be upset over Hyde Park. And in later life when he retired he'd go over the woods with the dog and sit on his favourite tree stump with the dog and bawl his eyes out.

JOHN MANN: There are far more lives that have been scarred and ruined by childhood abuse. Let the truth come out.