Almost exactly three years ago, Tom Watson stood up in parliament and demanded the Metropolitan police investigate ‘clear intelligence suggesting a powerful paedophile network linked to Parliament and No. 10’. It was an incendiary claim which, because it was made during Prime Minister’s Questions and broadcast on live television, set hares running on social media and beyond.

We know, now, that the police found no evidence to support an allegation of rape made against Leon Brittan by a woman known as ‘Jane’. But the question remains: what about that link to No. 10?

I have spent much of the past three years looking into this. Working for BBC Panorama means following the evidence — no matter how tough and painful the process.

One thing was clear straight away: the voices of the genuine survivors of childhood sexual abuse need to be properly heard. But the more I investigated the supposed VIP abuse network, the less I found. Slowly, another story started to present itself: how the Metropolitan Police had called a series of extraordinary high-profile investigations based on evidence that ranged from thin to nonexistent. The police service, which is supposed to be utterly impartial, appeared to have allowed itself to have been diverted by a frenzy to investigate.

Our Panorama documentary — ‘The VIP Paedophile Ring: What’s the Truth?’ — was broadcast last week. The Met were furious, claiming even before we broadcast that they had ‘serious concerns’ about its impact on the witnesses involved and on the ‘willingness of victims of abuse to come forward to police’. But soon the Met had to focus more on the consequences of its own actions. It has now belatedly apologised to the family of Leon Brittan for not making clear it had found no evidence to support ‘Jane’s’ rape allegation. But it has yet to say anything about what progress, if any, was made investigating the central allegation that has led to the current maelstrom.

What Tom Watson said three years ago was quite precise. He claimed that the evidence file used to convict Peter Righton (founder of the Paedophile Information Exchange) contained ‘clear intelligence of a widespread paedophile ring’. He said that one of its members boasted of his links to a senior aide of a former prime minister and that ‘the leads were not followed up’. It was this lead which, Watson had been told, led straight to 10 Downing Street, the highest office in the land.

Watson’s source was Peter McKelvie, a whistleblower with a long experience of working in social work and child protection. He had helped on a 1994 BBC documentary, The Secret Life of a Paedophile, which profiled Righton and brilliantly exposed how he had been allowed to rise to the top ranks of social work despite having admitted a sexual interest in children to colleagues. McKelvie claimed that evidence recovered by police from Righton’s house contained the supposedly explosive link to No. 10. McKelvie believed he had a lead to the former senior aide being prepared to look after child -pornography.

But who was this ‘senior’ politician? Watson was quick to point out who it wasn’t, ruling out Peter Morrison, a former aide to Margaret Thatcher. I soon established whom McKelvie believed it to be: a man who is now today a government minister. I won’t name him because, as we have seen over the past few months, baseless accusations against innocent men can cause permanent reputational damage. Mr Watson did not, evidently, believe these claims to be baseless — indeed, when I made my inquiries, I was also told that two witnesses would be able to confirm Minister X’s involvement. But when I tracked down the supposed witnesses, both told me that he never been part of the abuse they had suffered.

Of course, one can argue that victims of sexual abuse might issue false denials, not wishing to relive their trauma. But as far as I could tell, there was nothing more to go on: the link to No. 10 did not stand up. And I can now reveal that the Met drew the same conclusion years ago. Within two months of Watson’s incendiary allegations, the police detective on the trail of Minister X found nothing incriminating within the newly recovered evidence. I have seen an email he sent, confirming that there was ‘no evidence of offending linked to [Minister X] within the files’. He went on: ‘…there is not any further material within the file to support the inference to any level of criminal complicity on behalf of [Minister X]’.

So the Metropolitan Police have known for ages that the ‘clear intelligence suggesting a powerful paedophile network linked to Parliament and No. 10’ was no such thing. So why haven’t they clarified this? Why let the cloud of suspicion and innuendo hover over this minister even now?

I believe the answer is partly fear. Fear of returning to the mistakes of the past — of being again accused of not taking a victim’s accounts seriously, or of allowing someone’s status to fend off an investigation. Watson’s intervention has revealed some of the complexities in unravelling paedophile networks. His campaigning has led to at least three convictions, including that of a former teacher and a Catholic priest; had he not campaigned with such energy, it’s unlikely justice would have been done. So there were guilty men — just not the senior politicians who had been said to be in the frame.

The Metropolitan Police were late in admitting to Leon Brittan’s widow that their investigation of ‘Jane’s’ rape claims had yielded no evidence. It is surely now time for them to make it clear that, contrary to what Mr Watson claimed, the evidence file contained no ‘intelligence’, clear or otherwise, of the suggested link to Parliament or No. 10.

Alistair Jackson is a journalist for the BBC’s Panorama.