Tony swimming in Lake Garda in the summer of 1936.

      They found a new flat on Randolph Crescent, in Maida Vale. Then Tony went off to Wales to visit his family. ‘It’s horrid not having you about to kiss every hour while your [you’re] tidying the place up,’ Stephen wrote to him. ‘But it is lovely to look forward to our life together again.’ He loved being with Tony. He wrote to Kirstein, ‘He is one of those lucky people who knows he needs love all the time and who can accept it.’

      Their existence as a couple was noticed by Virginia Woolf. ‘I see being young is hellish. One wants to cut a figure. He [Stephen] is writing about Henry James and has tea alone with Ottoline [Morrell] and is married to a Sergeant in the Guards. They have set up a new quarter in Maida Vale; I propose to call them the Lilies of the Valley. There’s William Plomer, with his policeman; then Stephen, then Auden and Joe Ackerly, all lodged in Maida Vale, and wearing different coloured Lilies.’ The reference is to Oscar Wilde, who was often depicted holding a lily.

      Stephen had met the South African novelist William Plomer while he was still at Oxford. Joe Ackerley went on to become the editor of the Listener, where he frequently commissioned articles from his friends. Virginia Woolf was right to detect something conspiratorial in the vision of all these male couples living openly together, but she was amused rather than shocked – though maybe there’s an element of snobbery in there somewhere.

      As soon as he thought he had a satisfactory draft, my father showed her a typescript of The Temple. Perhaps it was his way of proving that this particular Lily of Maida Vale was serious. She gave it her full attention on more than one occasion, but she did not think that it worked. The Temple includes a sex scene between two men, but that was not the reason why she told him the book should be abandoned. Virginia Woolf was not shockable in that way, however sensitive her literary persona might seem. Sex was an aspect of freedom; therefore it should not suffer any puritanical restrictions. As a subject, however, sex disturbed her. My father told me that he’d asked her once how important in a relationship she thought sex was. She said: ‘It depends how highly you value cocks and cunts.’ She had not dodged the question. She’d merely killed the subject.

      In the spring of 1934, Stephen and Tony travelled to Yugoslavia via Venice and Trieste. By the middle of the month they were settled in a pension in Mlini, a small fishing village not far from Dubrovnik. The hotel’s other clients were Nazi holidaymakers, but apart from that, it was a peaceful place where Stephen could work.

      One day, there turned up at their hotel an American woman with her daughter, plus her daughter’s nurse. The woman was Muriel Gardiner, a young psychiatrist studying in Vienna with a pupil of Freud. She and Stephen made friends. She told him about the suppression of the workers in Vienna early in the year. She’d been there. She could provide an eyewitness account – which Stephen incorporated into Vienna, a long poem that tries to blend politics and personal experience into one narrative, in order to make the point that in this particularly dark period of Germany’s history private lives and public disasters were becoming inextricably tangled.

      In Vienna a few weeks later, while Tony was recovering from an operation for appendicitis, Stephen and Muriel began to have an affair.

      To Christopher Isherwood he wrote: ‘I find the actual sex act with women more satisfactory, more terrible, more disgusting, and in fact more everything. To me it is much more of an experience, I think, and that is all there is to it.’ This draws a line between himself and Christopher, who’d never had such doubts about his sexual identity. But there’s a curious aspect to ‘normal’ sex. ‘The effect is funny, because I find boys much more attractive, in fact I am more than usually susceptible.’

      This is honest, but it’s also strange. He loved Muriel, but a magnet drew him in the other direction. And Tony? Wasn’t he the steadiest relationship at the moment?

      He wrote about his confusion to William Plomer: ‘As a “character” I am no good. I realized that when I saw that I was capable of feeling just the same about Tony, & being attracted by a woman, and wanting to go with miscellaneous boys. Obviously if I’m like that my relationship with Tony becomes the thing that is most holding me together and that I must most cling to; and Tony becomes the person whom I get to appreciate more and more. Beyond that there’s really nothing but work, & pornography, silliness of all sorts.’

      Muriel observed this confusion sympathetically and without creating a scene. She was a psychiatrist trained to observe, not to intervene. She loved Stephen and she did not wish to challenge his ‘ambivalence’, as he called his obsession with Tony whenever he discussed it with her.

      As far as Stephen was concerned, Tony’s reaction was more important than Muriel’s. ‘Tony has been terribly upset about this, although he was extremely generous about it, and never felt in the least resentful. In fact, he was very fond of her.’ Tony was anxious that Stephen might want to get rid of him; and Stephen understood. He reassured Tony. ‘Even if I married, it wouldn’t form a separation, because we would want to live together for our whole lives.’

      His relationship with Muriel faded because chance and politics prevented them from spending more time together. Then Stephen suddenly decided that he could not go on living with Tony. ‘We had come up against the difficulty which confronts two men who endeavour to set up house together. Because they are of the same sex, they arrive at a point where they know everything about each other and it therefore seems impossible for the relationship to develop beyond this.’

      Two men living together formed ‘a substitute relationship in which each of the two expects the other to be something other than they are, because each of them is a substitute for something else’. Instead, a man and a woman living together could map out an area of reciprocal incomprehension that would allow their relationship to grow.

      It’s surely odd to suggest that heterosexual relationships work because they are based on mutual incomprehension, but he never went back on this view. It was even more odd that my mother, years later, defended this idea. I remember discussing it with her long after Dad’s death. She told me: If you are two people of the same sex, each will know how the other is feeling just by the shape of your companion’s shoulders as he (or she) gets out of bed in the morning. And, consequently, how the whole day will go. Whereas if two people of the opposite sex live together, there will be things that you’ll never understand about each other, however close you become.

      Mum told me this without seeing the absurdity of the idea. I wondered if Dad hadn’t been trying in his mild way to explain away his preference for men with a joke. I also wondered how she’d have felt if she’d ever interpreted his idea that heterosexual love is based on incomprehension as a rejection of herself; but as far as I know, this never happened.

      In World within World, he suggests that the attraction of working-class men was so unusual, it constitutes a gender of its own. ‘The differences of class and interest between [Tony] and me certainly did provide some element of mystery, which corresponded almost to a difference of sex.’ There are men, and there are women – and there are working-class men who are somehow in between?

      I hear the ghostly voice of Tony murmuring in my ear, telling Dad: Steve, you’ve gone daft!