If there is one event in the history of the 1930s that everyone knows, it is the crackly broadcast from Windsor castle of King Edward VIII giving up the British throne because he could not continue "without the help and support of the woman I love".

But if that woman, Wallis Simpson, had been Wallis Solomon, as well she might have been, would there have been an abdication crisis? Would a Mrs Solomon ever have been granted the kind of access to royal circles that would allow a relationship with the then Prince of Wales to begin in the first place?

In 2007 I started researching my biography of Wallis Simpson, a woman dubbed a Nazi spy, gold digger, and prostitute who learned her skills in Chinese brothels. But who was the real Wallis?

I decided to start with a key protagonist in the drama - Ernest Simpson, the husband she left for Edward, Ernest who is always portrayed as the traditional cuckolded husband about whom everyone has a good laugh behind his back. So I googled him. And there, at the end of the article, was a reference to a son born in 1939, christened in the Guards Chapel as Henry Child Simpson but who, since 1958, went by the name of Aharon Solomons, having added an "s" to the family name. This man was a free-diving instructor - teaching breath-held underwater diving - who had trained in Eilat but now lived in Mexico.

I called a number on the free-divers' website and, an hour later, got through to a friendly, upper-class English voice with Israeli inflections. The man was shocked at being tracked down. No one had ever interviewed him about Ernest and Wallis before. But we chatted and he told me that, although normally busy running his diving camp, he was about to have a foot operation and would be recuperating for two weeks. If I could come immediately, he would see me.

Looking back now I am shocked at myself for agreeing so readily to stay in the flat of a strange man. I had made cursory checks with a friend who had been at Harrow School at the same time as young Henry. Yes, he remembered the boy, he told me. But that memory was of little help, I reflected, as we set off one breathlessly hot afternoon from his flat to see his desert camp, travelling in an old pick-up truck with a machete (to kill snakes, he insisted), enough water for four days and not a human being in sight. The trip was necessary, he persuaded me, if I was to understand how stultifying was the lifestyle in England he had rebelled against. And so, during the course of the next few days, as we cooked on an open fire and slept on a makeshift table (high enough to avoid snakes), I began to unravel the strange tale of Ernest Simpson, the father of my Mexican diving instructor.

Ernest was born into an observant Jewish family of Hamburg shipping traders, who a mere generation back, had settled in Plymouth. Ernest's father broke away, changed his name from Solomon to Simpson on moving to New York, where he married into a Waspish family and gave young Ernest at 21 a choice of being English or American. Ernest chose England and joined the Coldstream Guards. He was fair with blue eyes, tall and good-looking and had no difficulty in leaving behind any trace of his Jewish origins. In those days the clubs and masonic lodges he wanted to join would not have accepted a Jew, so he hid his identity, even from his son, absorbing the mild anti-semitism of the then British establishment.

Ernest remained married to Wallis from 1928 until the famous divorce in 1937. Then he married Wallis's best friend, Mary Kirk Raffray, and Henry was born in 1939. But two years later, Mary died of cancer and the baby was sent to live in America with friends. By the time he returned to England, Ernest had married again and young Henry was packed off to boarding school where he was teased for being the son of Ernest Simpson. Yet he knew neither of his parents nor the scandal surrounding their lives until, in 1958, Ernest died of throat cancer and his aunt, Maud Kerr-Smiley, Ernest's older sister, decided it was time the unhappy teenager knew the truth about his Jewish heritage.

"I believe Maud wanted to spite her younger brother," Aharon explains. "But suddenly I felt I belonged somewhere. I had always felt neither fully English nor American, but I could become Israeli. I changed my name to what I thought was the family surname and went to live in Israel." Aharon immediately joined the IDF and in 1973 was fighting in the Golan Heights. Two sons, Uri and Nadav, who still live in Israel, have also served in the IDF.

For five days we discussed what sort of man Ernest must have been to have fallen in love with Wallis and then condone her relationship with the Prince of Wales. "He wanted nothing more than to be considered an English gentleman," his son says. "This meant not only behaving in a gentlemanly way by never insulting a lady, but wearing his Guards tie most days. He had a fine collection of antiquarian books and was happy spending an evening in his study reading Latin and Greek."

Wallis was the party animal and Ernest, so in love with her, tried to give her what she wanted. But he was also deeply in awe of the British monarchy so that even after the abdication, even after he had lost his wife to the King, Mary, his third wife and Aharon's mother, wrote in her diary: "Ernest still thinks the Windsors are perfect".

Aharon, who had never known Wallis herself, gave me various introductions to his extended family. Over the next months I made trips to cities around the world in my quest to find them. On one of these, nearly two years after my trip to the Mexican desert, I was shown a cache letters and diaries that dramatically changes the traditional interpretation of the abdication crisis. But, had she been Mrs Solomon, Wallis's relationship with Edward would probably never have started.

'That Woman: The Life of Wallis Simpson' Duchess of Windsor by Anne Sebba is published by Weidenfeld & Nicolson at £20.00. 'Wallis Simpson The Secret Letters' is on Channel 4 on Wednesday at 9pm