The former officer who handed it over, Dave Flanagan, told the BBC he believed the list may originally have been seized in a police raid in the late 1970s.

The document given to the BBC has scribbled notes in the margins – and Mr Flanagan, a detective constable at the time, says he wrote some of them.

He also attached and dated the pink cover page – as he and colleagues added more up-to-date PIE intelligence during the 1980s.

Police raided plenty of people on the list, he says – but, on its own, it was unusable as information for a search warrant.

“You couldn’t go in front of a magistrate and say: ‘Look, we believe he’s a paedophile. We believe there’ll be indecent photographs of children because he’s on the PIE list.'”

Legally, being a member of a pro-paedophile group didn’t make someone a sex offender.

Police did manage to close in on PIE in the early 1980s – focusing on three senior members who all had links to contact adverts in the members’ magazine, Magpie.

The men were prosecuted under a 17th Century law of “conspiracy to corrupt public morals”. Two received conditional discharges, while the third was jailed for two years.

Publicly, PIE ceased to exist in 1984.

Dave Flanagan says his team’s detective work on the membership list also ground to a halt.

“Information was passed to other police forces and they did what they did with it – we had no control over any of that.”

The BBC understands the PIE list was digitised in 1994 by a police team that no longer exists. The National Crime Agency, which was formed in 2013 and whose officers deal with child abuse cases, told us it has “no knowledge of receiving the [digitised] list”.

Dave Flanagan kept the original in his briefcase until he retired in 1998, when he handed it to Peter McKelvie.

Mr McKelvie told the BBC that over the past 30 years he had pushed police, a Labour MP and a Conservative government minister to look at PIE members linked to social services and special schools, but without success.

He wrote to the Department of Health in 1993 outlining his concerns. His letter began: “The infiltration of the social work profession by paedophiles appears to be an extensive and serious problem…”