piers morgan princess diana
The first time I met Princess Diana, at a charity event in London when I was editor of the Daily Mirror newspaper, she marched over to me and declared: 'Ah, the man who thinks he knows me so well!'

I suggested she take this golden opportunity to enlighten me on what I should know, and she burst out laughing. 'I don't have the time...or the inclination, come to that!'

She then glided on, working the room as only she could, melting everyone in her wake with her dazzling beauty and charm.

Diana was one of the most fascinating, complex, passionate, sexy, scheming and unpredictable women I have ever met. She was also utterly fabulous.

Last night a stunning new documentary aired in England, containing extraordinarily candid interviews with her two sons, Princes William and Harry.

Diana: Our Mother, which airs on HBO in America tonight, was as sad and poignant as it was riveting and popular - over seven million Brits tuned in to watch.

But amid all the obvious continued heartache following her tragic death in a car crash 20 years ago next month, there was also a reminder of what a fantastically funny and entertaining creature she was too.

'One of her mottos to me,' said Harry, 'was "you can be as naughty as you want, just don't get caught". She was one of the naughtiest parents.' You can say that again.

Two months after that first encounter, Diana invited me to have a long lunch with her at Kensington Palace. By that time, May 1996, she was the biggest star in the world. Perhaps the biggest star the world has EVER known, the greatest celebrity of a celebrity age. A woman who sent even the likes of Michael Jackson giddy with excitement.

So I found myself feeling unusually nervous as I arrived, and even more so when she asked: 'Would you mind awfully if William joins us?'

What followed remains one of the most surreal experiences of my life.

Diana was staggeringly open with me, about Charles, the Queen, and the many varied men in her life.

It was clear that she found her ridiculous global fame both exciting and terrifying. 'I sometimes dream of emigrating,' she sighed at one stage, 'but to where? Somebody would find me wherever I went.'

That was true.

As her brother Charles said at her funeral: 'Diana, a girl given the name of the ancient goddess of hunting, was the most hunted person of the modern age.'

princess diana
Yet part of her loved the hunt, and the thrill of the chase - even with the paparazzi.

That streak of naughtiness she encouraged in her sons was very evident during the lunch when William, then just 13, asked at one stage: 'Can I have some wine, Mummy?'

'No William, whatever are you thinking?' she replied, with an indignant face of faux horror.

'But Mummy, I drink it all the time,' he retorted. 'Er, no you don't,' she snapped, and you can't have any...'

'Yes I do, and yes I can,' he chuckled with a mischievous grin.

Two months after that lunch, I myself got into huge trouble after publishing a rather jingoistic war-flavoured entreaty to the England football team to beat Germany in the European Championships.

I bumped into Diana again at London's Royal Brompton heart hospital where she was meeting some patients. 'Well, well, Mr Morgan,' she giggled, 'haven't we born a naughty boy! Thanks for keeping me off the front pages....'

She seemed highly admiring of my shameful conduct.

Then I watched her interact with the sick kids, walking from bed to bed, hugging and cuddling them in a way that made them hug and cuddle her back. Professor Magdi Yacoub, the world famous heart surgeon, was watching with me. 'Look at Diana,' he said, 'she's like a living saint, isn't she?'

In many ways, she was. Diana had an absolute heart of gold, no question.

Her butler Paul Burrell told me how she'd often escape the Palace in disguise late at night to go and hang out with homeless people sleeping rough on the streets of West London.

But she was also very complicated, determinedly and recklessly man-hungry, and occasionally downright duplicitous, especially in her dealings with the media.

I suffered the latter at first hand when, a little while after our lunch, Diana made a secret visit to a private clinic where she spoke to some patients about her own battles with eating disorders.

princess diana
She was frustrating and complicated as well. She would leak information to the media (and me), then condemn the stories printed
One of those patients called the Mirror to relate what Diana had said because she found it so inspiring and wanted other sufferers to hear it. Due to our newfound relationship, I phoned Diana to tell her and, at her request, faxed through the copy.

She then rang me and gave me a one-hour rundown of exactly what had happened, so I could get it right. I told her I was taping the call. I then faxed the revised version over to her later and her office rang to thank me and say she was 'very happy and very grateful'.

So we published our front-page scoop, without directly quoting Diana or saying she had spoken to us.

The next morning, Diana issued a furious statement ferociously condemning the Mirror story, and saying she was 'deeply disappointed to learn that her 'private conversations' had been disclosed to a newspaper.

I rang her office, equally furious, and warned I may put the tape of her call to me on a phone line in the next day's paper. Her aide took muffled instructions.

'The Princess doesn't think you will do anything like that now you are getting on so well,' came the eventual response. And she was right.

I was now trapped on Planet Diana, a crazy place where she called all the shots and behaved how the hell she liked, oblivious to the consequences.

I spoke to her quite frequently in that final year of her life, so was able to witness at first hand just how manipulative she could be. A month before she died, Charles threw a 50th birthday party for Camilla Parker-Bowles.

At the time, Diana was holidaying in the South of France with billionaire Egyptian tycoon Mohammed al-Fayed.

He rang me early on the morning of the party to suggest it would be a good idea if our photographers were near the beach outside his villa. I could hear Diana in the background, instructing him on what to tell me.

At 9.30am, she appeared on the beach in a leopard-skin swimsuit and posed, preened and performed cartwheels for the next half an hour.

She knew these photos would knock Charles and Camilla's big night off the front pages, and they did.

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Al-Fayed rang next day to thank me. 'Diana is so happy!' he said. 'She says a big thank you!'

A few days later, news broke of her new romance with Mohammed's son Dodi, one that was to end with them both dying in Paris, as they were pursued by paparazzi.

I felt I'd known Diana for my whole journalistic career, such was the impact she had on the news cycle and newspaper offices during that time. In reality, I'd only really known her for little over a year. But that was long enough to realise she was a quite extraordinary human being.

One who was capable of enormous empathy and kindness, yet also witheringly brutal disdain and callousness, sometimes in the same conversation.

She cut friends off, feuded with family, hopped from one bed to another like an over-sexed reality TV star, and was in a permanent state of love and hate with the media. But she also lit up the world and put a detonator to the very foundations of the British Monarchy.

Many worried the royals wouldn't survive her death. But they have, and aside from the wondrously consistent and calming majesty of The Queen, that is mainly down to her sons.

Diana single-handedly dragged a very stuffy institution kicking and screaming into the modern touchy-feely world.

William and Harry are now ensuring her legacy is protected, remembered and nurtured, and the Monarchy isn't allowed to slide away into overly formal irrelevance.

The soul-searching way they have recently taken to pouring their hearts out about everything from mental health to the death of their mother may not be everyone's cup of tea. In fact, I'll readily admit it's not mine.

But it perfectly resonates with most of the social media driven youth of today, who find the Princes both relatable and, crucially, relevant.

I found the documentary profoundly moving; a fitting tribute to an incomparable woman cut off in her prime. I also found the two princes incredibly impressive.

Diana, I'm sure, would be bursting with pride at the way William and Harry have turned out, albeit whilst quietly urging them to carry on being as naughty as hell.