The interview was infamous, made his name and was bound to enrage. It also received a viewing audience of 23 million people who heard a saucy tale of adultery, plots in the palace, and stories of physical and mental illness. But the tarring and feathering of Martin Bashir for his 1995 Panorama programme featuring Princess Diana was always more than the scruples of a journalist and his interviewing methods.

Princess Diana’s brother, Charles Spencer, eternally committed to excoriating press coverage of his sister, brought pressure to bear on the BBC last year in allegations that Bashir had used forged bank documents to induce the interview. The documents suggested that people were receiving cash to monitor the princess. Bashir, Spencer accuses, sowed the seeds of concern that a conspiracy within Buckingham Palace was afoot against his sister. Bashir, for his part, claimed that the documents in question had “no bearing whatsoever on the personal choice of Princess Diana to take part in the interview.” For Diana fans, such a detail is irrelevant.

In November, the BBC appointed retired Supreme Court judge Lord Dyson to revisit old ground, notably the 1996 investigation by Tony Hall, who found Bashir to have been “honest” and an “honourable man”. Lord Dyson begged to differ. “What Mr Bashir did,” the report states, “was not an impulsive act done in the spur of the moment. It was carefully planned… What he did was devious and dishonest. To dismiss his actions as no more than a mistake, unwise and foolish did not do justice to the seriousness of what he had done.” Hall’s investigation was “flawed” and “woefully ineffective”. The BBC had fallen “short of the high standards of integrity and transparency which are its hallmark” effectively instigating a cover up.

Figures have queued up in a feast of Bashir bashing. John Birt, Director-General of the BBC at the time of the interview, called the journalist’s conduct “a shocking blot on the BBC’s enduring commitment to honest journalism; and it is a matter of the greatest regret that it has taken 25 years for the full truth to emerge.”

Prince William and Prince Harry have unsurprisingly joined the fray for their mother’s memory. In his statement, the Duke of Cambridge accused the BBC of contributing “significantly to [Diana’s] fear, paranoia and isolation that I remember from those final years with her.” She had been failed by both “a rogue reporter” and “leaders of the BBC who looked the other way rather than asking the tough questions.” Prince Harry looked beyond the conduct of the public broadcaster, seeing it as part of “the ripple effect of a culture of exploitation and unethical practices ultimately took her life.” Harry, it should be noted, is no fan of broad free speech protections. “I’ve got so much I want to say about the First Amendment as I sort of understand it,” he told actor Dax Shepard, the host of the Armchair Expert podcast, “but it is bonkers.”

Lord Dyson’s report, as with the Leveson Inquiry into the phone hacking conducted by such press outlets as the now defunct News of the World, risk becoming springboards for a broader agenda of censorship and press control. The latter’s suggestion that the press be subjected to an “independent” and “voluntary” regulator created by statute caused alarm and did not sway the then Prime Minister David Cameron. As Index on Censorship warned in 2013, “Any law which sets out criteria that the press must meet, by definition introduces some government or political control of the media.”

The press regulation agenda ignores the fact that journalism as practice can, even should, be unethical if the broader public interest demands it. Going under cover, masking identities, concocting fables may be necessary to expose misconduct and villainy. “There are cases, and undercover is one of them,” stated former Panorama editor Tom Giles to the House Communications Committee in 2012, “where technically, we break the rules. Technically we break the law whether it is on privacy or on giving a misleading CV in order to ensure that we are able to go undercover.” For Giles, there had to be “very clear prima facie evidence that this is something that is of significant public interest.”

The Communications Committee also heard similar testimony from Chris Birkett, Deputy Head of News and Executive Editor of Sky. “For us, there are times when the only way to get the story is to do something that is contrary to the laws of the country in which we are doing journalism.” His example: reporting on Syria. “If you try to film openly, you will be beaten up and arrested, your camera will be smashed and you will be put into prison.”

This is not to say that a journalist has an inherent right to break the law. Bashir might well have been prosecuted for forgery. The issue is one of degree. Nick Davies, who exposed the phone-hacking scandal in the pages of The Guardian, suggested that “all citizens have a right of conscience in extremis to say, ‘This is so important I’m going to break the law’.”

The question as to whether knowing the personal affairs of Princess Diana was in the public interest, or merely interesting to the public, will never go away. This was not the issue of exposing the handiwork of war criminals, or the shady transactions of an underworld figure. Bashir’s interview style and approach has always had a certain tabloid flavour. But reporting on the activities of a modern constitutional monarchy is bound to blur the line. According to Tessa Clarke, herself a former BBC Panorama reporter, “Bashir’s methods were neither exceptional nor unjustified – the marital affairs and behaviour of the heir to the throne is in the public interest.” People do want to know what the royals are up to, while the royals are keen to be talked about as long as they control the narrative.

The eternal flame of the Diana Cult is one that constantly threatens purges and censorship. Only hagiographers are welcome to the shrine. Prince William is keen to take the purging further. The interview, he demands, should be scrubbed from the historical record. “It is my firm view that this Panorama programme holds no legitimacy and should never be aired again.” The concern now is how far that purging will go in the battles over what can, or can’t, be reported.