When Jimmy Savile died in October 2011, stories about his murky past resurfaced but, despite the best efforts of certain journalists, it was not until a year later that ITV finally revealed the full horror of the claims against him.

Among new details to emerge during the ensuing public outcry was that two police forces – Surrey and Sussex – had investigated the broadcaster and prolific charity campaigner between 2007 and 2009 after four women made accusations against him.

They had done so under the auspices of the Crown Prosecution Service, run by Keir Starmer since 2008. It provided regular guidance to detectives before concluding in October 2009, the same month Savile was interviewed by Surrey Police under caution, that no prosecution could be brought because none of the four complainants was willing to support police action and the evidence against him was weak.

This was probably the last chance for justice for Savile’s victims and, in what ranks as one of the biggest blunders in recent criminal history, it was missed.

Starmer’s story is that he, personally, was never aware that Savile had even been accused, let alone that the CPS had reviewed allegations against him and decided he could not be charged – even though he had been the Director of Public Prosecutions (DPP) for nearly a year by the time that now highly controversial decision was made.

“It never came close to crossing my desk and the local CPS lawyer who looked at the case did not even mention the decision to his immediate boss because, to him, it seemed routine,” he has said.

But does this explanation hold water? On October 24, 2012, in the wake of the floodgates being opened against the shamed broadcaster by the ITV documentary, Starmer acknowledged that the CPS had apparently allowed Savile to slip through the net. He said he had asked the chief crown prosecutor for the south-east, Roger Coe-Salazar, to “consider the files” in relation to the four incidents referred to the CPS by the police in 2007 and 2008.

Starmer said Coe-Salazar had “assured” him that the decisions that were subsequently taken were the right ones based on the information and evidence then available.

In leaving it to Coe-Salazar to make this assertion, Starmer showed he was perfectly happy for the CPS to mark its own homework. But Starmer also said: “Out of an abundance of caution, I have asked for the papers in the four cases to be provided to my principal legal adviser, Alison Levitt QC, forthwith so that she can consider the decisions made and advise me accordingly.”

Starmer was therefore content for the CPS to mark its own homework for a second time – this time with Levitt wielding the red pen. Levitt’s report was published in January 2013. It provided some worthwhile insights but also posed new questions that remain unanswered.

Levitt found that, in May 2007, a complaint was made to Surrey Police alleging that in the late 1970s Savile had sexually assaulted a girl aged 14 or 15 at Duncroft Children’s Home in Stained, Surrey. During a subsequent investigation by Surrey Police, two more claims surfaced. One, dating from about 1973, accused Savile of sexually assaulting a girl aged around 14 outside Stoke Mandeville Hospital. The other, again from the 1970s, was that Savile had suggested to a girl of about 17 – also from Duncroft Children’s Home – that she perform oral sex on him.

Separately, in March 2008, Sussex Police had looked into a claim that, in about 1970, Savile had sexually assaulted a young woman in her early twenties in the back of a caravan in Sussex. In the course of their respective investigations, detectives from each police force came to know of the allegations the other was looking into.

Levitt explained that an “extremely experienced” CPS reviewing lawyer had supervised the Savile matter when it was brought to the CPS’s attention. Her report gave the impression this lawyer oversaw the case alone. She interviewed the lawyer for her report, but granted him anonymity – a decision Starmer does not appear to have queried.

To this day, his identity remains secret.

All that is known about him is that he worked in the CPS south-east office and had retired from the CPS by January 2013. Further enquiries for my book have established he was a special casework lawyer in the rape and serious sexual offences unit (RASSO), a small division that mainly operated from offices in Guildford.

I shall refer to him as the “mystery lawyer”.

Significantly, Levitt stated that the mystery lawyer “struggled to remember the details [of the case] after all this time”. It is surprising that Levitt was not more sceptical about his faulty memory of what was always going to be a high-profile case.

But in view of his poor recollection of events, it is difficult to avoid wondering how reliable he was as a witness. Furthermore, Levitt does not appear to have asked him to explain whether he ever discussed the Savile matter with any of his CPS colleagues – including Starmer – over the 30 months that he took charge of the case.

Police records show the mystery lawyer held three meetings with Surrey Police to discuss the case against Savile. The first was on July 15, 2008, 15 weeks before Starmer became DPP. The next was on January 22, 2009, almost three months after Starmer became DPP. The last was on March 31 – a full five months after Starmer became DPP.

Savile was interviewed by Surrey Police under caution at Stoke Mandeville Hospital, where he was a volunteer, on October 1 – 11 months after Starmer became DPP. During this interview he denied the three allegations put to him and told officers the complainants were after his money.

The mystery lawyer had a telephone meeting with Surrey Police a week later, on October 8. He then provided final written advice to the police on October 26 in which he said: “On applying the evidential test, in the absence of statements from the victims, there was insufficient evidence to charge with any criminal offence.”

On October 28, a police officer (whom, again, Levitt did not name) wrote to the four women involved telling them: “The CPS have decided no further police action on this case.”

Levitt’s report stated that the case files were returned to the police after it was decided no prosecution would take place. She explained that all traces of the Savile file were removed from CMS [the CPS’s internal electronic case management system] and “destroyed” on October 26, 2010 – two years after Starmer became DPP.

This apparently happened in accordance with data protection policy. Since it was impossible to retrieve the CPS’s own Savile file, some will wonder which “files” Starmer referred to when he made his own statement declaring that his CPS colleague Roger Coe-Salazar had given the CPS a clean bill of health.

Levitt’s report concluded that the decision not to prosecute Savile was made in good faith but that the police and the CPS had been overly cautious in their handling of the matter. Most egregiously, Surrey Police did not tell each complainant of the other complaints that had been made. And Sussex Police told its complainant that corroboration would be needed in order to charge Savile.

Levitt did say she thought the mystery lawyer showed a surprising lack of curiosity when told by police that the complainants did not support the prosecution.

She queried why he did not try to build a case. She also noted that had the complainants been given more information by the police at the time of the investigation – and if each one had been told that she was not the only woman to have complained – they would probably have been prepared to give evidence.

The inference was that Savile could have ended up in court had the police and the CPS handled things differently.

Savile’s assumed victims were expected to accept this slipshod approach and move on. So was the British public. But the fact is that two major police forces investigated Savile for more than two years with the knowledge of the CPS and, in the case of Surrey Police, with the advice of the CPS both before and after they interviewed him under caution.

How likely is it that Starmer remained completely oblivious to this inquiry?

We know it was active for the first year that he was DPP.

Savile was not some unknown figure. He was one of the most famous men in Britain, a stalwart of the BBC and the charity world, a knight of the realm, and a sometime friend of the royal family who had once been trusted by senior politicians including Margaret Thatcher.

He was, surely, a very likely subject of discussion within the higher echelons of the CPS given the nature of the allegations against him.

How many CPS employees, other than the mystery lawyer, knew that the CPS was looking into this highly sensitive case between 2007 and 2009? Levitt’s report did not say.

Equally, who decided that Starmer, who had made a point of telling staff when he became DPP that his door was always open, and who attended regular meetings with CPS staff around the country, should be kept in the dark about the Savile investigation?

It isn’t even clear whether he first learned about the CPS’s involvement before or after Savile’s death. Levitt, later made a Labour peer by her former CPS boss, apparently failed to ask these basic questions. This turned out to be convenient for Starmer, but the public remains unsure about this undeniably murky business.

What is known thanks to Levitt’s report is that entities other than the CPS [open italics] were [close italics] aware that Savile was under investigation. Between 2007 and 2009, Surrey Police informed Surrey County Council’s children’s services, the charity Barnardo’s and West Yorkshire Police child protection unit that Savile was under suspicion.

Again, many will find it curious that the police volunteered to each of these organisations what was going on but that somebody at the CPS considered it best that Starmer, the most senior prosecutor in the land, should remain ignorant.

If Starmer really didn’t know what was going on at the CPS at the time, why wasn’t he better informed? It is a question that continues to stalk the Prime Minister to this day.

Adapted from Red Flag: The Uneasy Advance Of Sir Keir Starmer, by Michael Ashcroft (Biteback, £16.99)