"Never complain, never explain." The phrase, coined by former British prime minister Benjamin Disraeli, is said to be an unofficial motto for the British royal family. Part mystique, part stiff upper lip.

But 25 years ago this weekend all that would change when Princess Diana made it clear she saw no strength in silence. The most photographed woman in the world decided to talk.

And when she did there was very little on which she held back. Her adultery, her husband's adultery, her battle with an eating disorder, Prince Charles' suitability for the throne. The princess chose to open the lid on the inner workings of the royal family in a way that had never been seen before, nor since.

It was by any measure the journalistic scoop of the decade, if not the century. The fact that the princess had agreed to talk to Martin Bashir of the BBC's Panorama programme was known by very few at the broadcaster in advance. It was filmed in secret in the princess' Kensington Palace home, and when it was revealed it was explosive.

Twenty-three million people tuned in that night to hear what Diana had to say. Debate raged not just around what she had said, but why she had said it.

Was it her ultimate revenge on 'The Firm'? On the cusp of divorce from Prince Charles, did she see it as her last chance to have her say before a settlement that might have involved a confidentiality clause? 

Prince Charles and Princess Diana on their last official trip together in Seoul, South Korea in November 1992

Twenty-five years later, and why and how the interview happened is once again making headlines. But this time it's the BBC that has questions to answer. The corporation has announced an inquiry into how Martin Bashir obtained the interview, amid accusations from Charles Spencer, Princess Diana's brother, that his sister was tricked into speaking on camera.

Earl Spencer has said that he was shown bank documents by Martin Bashir that suggested both he and his sister were being spied upon. He has also suggested that there was "other deceit" involved in persuading him that an interview would be a good idea. The earl has said that this was a factor in his introducing his sister to Mr Bashir, which eventually led to the interview.

However, the bank documents were later shown to be fake. The graphics expert who drew them up at the behest of Martin Bashir has said that he did not know how they would be used, or that they would be shown to anyone. He also said that when he raised his concerns with BBC bosses, work with the corporation dried up and he believed he had been ostracised. By then, of course, the interview had been broadcast, and had drawn headlines and praise across the world. 

Martin Bashir

The allegations surrounding the documents were revealed in a tabloid newspaper in 1996 and an internal BBC investigation was ordered. The findings of that initial inquiry suggested Martin Bashir had been "incautious" in relation to the bank statements but deemed him to be an honest man.

The BBC also said it possessed a letter from Princess Diana, saying she had not been coerced in any way into doing the interview.

Martin Bashir left the BBC in the years afterwards, moving on to work in a number of high profile roles for other broadcasters. But he returned in 2016 as the BBC's Religion Editor.

Earl Spencer

He is currently on sick leave from the organisation, recovering from coronavirus and heart surgery, and so has been unable to provide any context to the current debate.

However, he is likely to find himself under pressure to answer questions quite soon, as the BBC has now begun another inquiry into the events surrounding the interview, headed by one of the UK's most senior judges, Lord Dyson. He has promised it will be "thorough and fair" and has estimated that it could take up to six months to conclude.

The BBC has promised the report will be published in full. Lord Dyson is now tasked with getting to the bottom of the ongoing concerns of Earl Spencer, who believes he was duped into persuading his sister to speak to Martin Bashir. 

The inquiry could take up to six months to conclude

The inquiry will also look at whether any breaches of the BBC editorial code occurred, and it will look at the internal investigation carried out in 1996, essentially investigating the initial investigation.

It is a complex tale going back 25 years, and there are some who may wonder at the necessity to dredge it all back up again. But there is a growing chorus of voices who want it fully investigated, the most recent of which is that of Prince William, Diana's eldest son.

A glance at the terms of reference for the new inquiry shows just how many questions the BBC feels need to be answered as a matter of urgency. Many are questions that go to the heart of its journalism. The inquiry will focus on:

1. What steps did the BBC and, in particular, Martin Bashir take with a view to obtaining the Panorama interview in 1995? This will include looking at the mocked-up bank statements, alleged payments to members of the royal household, and other issues raised by Earl Spencer.

2. Were those steps appropriate, particularly in regard to the BBC's editorial standards at the time?

3. To what extent did the actions of the BBC and, in particular, Martin Bashir influence Diana's decision to give an interview?

4. What knowledge did the BBC have in 1995 and 1996 of the relevant evidence, such as the forged bank statements?

5. How effectively did the BBC investigate the circumstances leading to the interview?

Debate raged not just around what Princess Diana had said, but why she had said it

Investigating events like this so many years later can be problematic. People have forgotten details or passed away in the intervening years.

Indeed, some might argue the only person who can truly say what persuaded Princess Diana to do the interview is the princess herself, and she died in 1997.

But the passage of time can also persuade some who didn't speak all those years ago to discuss what happened, giving potentially valuable new insights.

As Benjamin Disraeli also said: "Truth is more precious than time."