OUTWARDLY, perhaps, it seemed like just another happy marriage.

Ken and Hazel Houston were an Edinburgh couple who had married in 1954. When Hazel accepted his marriage proposal after a brief courtship, he was so "gloriously happy" that, after a taxi ride along Princes Street, he impulsively gave the driver double the fare.

When they married Ken believed that he had managed to put aside his secret feeling that he was transgender. But, unable to do this, he eventually told Hazel, in 1958. The disclosure came as a profound shock to her, but the marriage managed to endure.

Ken had apparently wanted to be female from the age of three. In his teens he made or collected girls' clothing and dressed as a girl when his parents were out.

In the 1970s he finally underwent oestrogen treatment, but he died in 1979, aged 60. Hazel died in 2003. They had no children. There story was forgotten ... until now.

Ken's story echoes The Danish Girl, Tom Hooper's much-praised film about the transgender artist Lili Elbe, who was one of the first people to undergo sex reassignment surgery and is played in the film by Eddie Redmayne.

The Houstons' remarkable story has now been told in a new book based on an archive of letters and photographs, inherited by artist Sara Davidmann and her siblings from their mother, Audrey, Hazel's older sister. Hazel confided heavily in her after learning of Ken's secret.

Audrey had labelled one of the envelopes in the archive 'Ken. To be destroyed' but despite this, could never bring herself to dispose of it. The book has the same title.

Davidmann said that she learned from reading her aunt's letters that Ken, an ophthalmic optician, believed he was "masquerading" as a man but was deeply uncomfortable at having to do so. By the late 1950s, he had begun to avoid public places and social interactions as often as possible. By 1963, however, the couple had managed to reach a compromise.

"We know that by then, Hazel had come to accept the situation," Davidmann added. "She says as much in a letter to my mother. What my mother told me was that Ken lived as a woman in the home but lived outside the home as a man, and went to work as a man."

The archive also contains letters from Professor Millar of Aberdeen Royal Infirmary's Psychiatric Department, whom Ken had consulted. In one letter Prof Miller tells him: "Your wife must, I think, accept the unusual and painful fact that her husband is more woman than man and likely to become so increasingly as time goes on."

Handwritten charts made in the 1970s document the changes to Ken's body as the consequence of oestrogen treatment.

Davidmann said: "If Ken had been alive now I think he would have been able to have lived a more open life."

Despite their early difficulties the couple remained very close to each other until the end.

"One of the important things for me about their story is that they stayed together," their niece says now. "It must have been very difficult at times, however. Hazel showed so much fortitude.

"It's a great shame that as a child growing up I hadn't realised how amazing they were. It's a great shame when you have those things that are kept from you. I wish I could talk to them both now. I wish I could know what they thought of the work I am doing."

The new book makes clear that the Houstons’ marriage was ultimately based on love and affection. Today they lie buried, side by side, in an Edinburgh graveyard.

* Ken. To Be Destroyed by Sara Davidmann, is edited by Val Williams and published by Schilt Publishing