The pictures, when they finally surfaced, rocked the nation – if not the friends who were in on the secret.

The apparently happily-married Princess Margaret, in figure-hugging corset-boned swimsuit, is nestled up close to a penniless toy-boy 18 years her junior.

He is wearing a pair of Union Jack budgie-smugglers she’d bought for him within minutes of their first meeting.

This is February, 1976. And she, in the 15th year of her marriage, is hopelessly in love.

Her sister, Queen Elizabeth II, knows all about it – and lets it be known she disapproves of Margaret’s ‘guttersnipe life’.

But theirs was a love affair which survived separation, nervous collapse and a suicide attempt. It lasted for seven long years and changed the face of royal marriages for ever.

Roddy Llewellyn, a lowly pen-pusher at the College of Heralds, was the man who made Princess Margaret happier than she had ever been.

Once, the princess had been second in line to the throne.

With her trademark hour-glass figure and permanent cigarette holder she was a sexy, wayward, dangerous, and imperious beauty.

Her 1960 marriage to bohemian photographer Tony Armstrong-Jones seemed the groovy way to go after her clandestine relationship with war ace Group Captain Peter Townsend collapsed in scandal and recrimination.

More than a dozen years in, however, the wheels were falling off.

Tony – given the title Lord Snowdon as a wedding present – was serially unfaithful from the outset. Margaret tried out a couple of lovers in retaliation.

But it was only when, in August 1973, she was invited by her lady-in-waiting, Lady Glenconner, to lunch at the family mansion in Scotland to find herself seated next to a floppy-haired 25-year old that the 43-year old princess truly fell in love.

It took less than a minute. It was crazy.

‘He was fascinated by this sophisticated, voluptuous woman, so amusing and so easy to talk to,’ wrote the distinguished biographer Anne de Courcy.

‘While she made it plain [from the very first moment] how attracted she was to him.’

Fascinatingly, the age-gap in the romance mirrored Margaret’s notorious affair with Peter Townsend – while he had been 16 years her senior, she was 18 years older than Llewellyn.

Roddy, said Margaret’s intimates, was tender and kind in a way Snowdon could never be.

On the other hand he did not have the bedroom staying-power of the older man.

Even as their marriage was falling apart Snowdon would stride naked across the corridor which separated their bedrooms and stand commandingly in front of his wife.

‘He would fling open my bedroom door, stand there with no clothes on and then – well, what could I do?’ Margaret told a friend later.

It was Snowdon’s sexual domination which made Margaret submit to him in other aspects of her life, claims Ms de Courcy.

It wasn’t the same with Roddy. Young, kind and attentive – the very antithesis of Snowdon – he began to quail at Margaret’s sexual demands after the initial honeymoon period passed.

‘He’d never had a long relationship with a woman before,’ explained Ms de Courcy. ‘Now he was involved with one who was not only far richer and older but who had a demanding, intense and possessive personality.’

According to Roddy’s elder brother Dai, the physical side of the relationship was proving very difficult to sustain.

But there is no doubt they loved each other. And three years into the relationship – still, surprisingly, a tightly-kept secret – the couple flew to the hideaway island of Mustique as ‘Mr and Mrs Brown’.

They revelled in the tiny Caribbean island’s exotic life and were able to act as if they truly were married – safe in the knowledge that prying eyes and long-lens cameras were unlikely to disturb their privacy.

They were wrong.

Someone took a photograph of them silhouetted against the backdrop of the ocean, quite clearly a loving couple oblivious to the rest of the world.

The picture appeared a few days later in a British red-top newspaper.

All hell broke loose. As far as the British public were concerned, Margaret and her husband – plus their two small children David and Sarah – were a tight-knit unit, living exemplars of the idea of a Royal Family.

In the day, it created as big a scandal as the Townsend affair – and the publication of the picture across the world caused Snowdon to demand a divorce.

He’d played the game, he argued, by keeping secret his extra-marital dalliances. If Margaret didn’t care who knew about her and Roddy, however, he was off.

It was at that moment the Queen’s former private secretary, Lord Charteris, let it be known that the sovereign’s opinion was that Margaret was living a ‘guttersnipe life’.

This was an extraordinary – some historians would say unique – insight into the relationship between Lilibet and her sister.

Roddy couldn’t face the pressure of publicity and ran away – first to the Channel Islands, then to Turkey, then South America.

Travelling on a bus, he let slip to a fellow passenger that he ‘had been having an affair with a married lady and that it had all got too much for him and the sex had become a problem.’

Margaret, bereft of the company of both her husband and her lover, went into a tailspin and took an overdose of Mogadon sleeping tablets.

Her recovery was feeble, cantankerous, and long.

Eventually, the lovers reunited. But Roddy, having escaped the claustrophobic atmosphere of the relationship, was looking for a new life – and found one, as a successful landscape gardener, author and TV presenter.

He inherited the family baronetcy – he is now Sir Roderic Llewellyn – and married, most successfully, Tania Soskin, the daughter of a film producer.

These may not have been the last words they shared, but they demonstrate just how a passionate affair between two people of vastly different backgrounds and ages can easily founder:-

Princess Margaret (modelling frothy pink dress that shows off her curvy figure): ‘What do you think, Roddy?’

Llewellyn: ‘You look like a strawberry ice cream.’