An academic victim of a violent attack in interwar London haunts Matt Houlbrook's exploration of a lost era's shifting sexual identities.

One night in August 1929, two lives met. Philip was American, a "scholarly" Harvard-educated professor of chemistry in his forties. He was in London for the summer and rented an apartment in Mayfair's "quiet and select" Half Moon Street. Roland was 22. He was from West Yorkshire and had worked in the local colliery before escaping the brutal realities of the Depression for the Coldstream Guards. Recently discharged and unemployed, he eked out a precarious existence around Hyde Park.

Philip was "taking the air". Roland was killing time, looking for something. On Piccadilly their paths crossed. The men exchanged glances, stopped, and talked for a while. The following evening Roland saw a friend at a nearby coffee stall. "I'm broke," he told her, "but I won't be tomorrow morning. I met an American. He's rolling in money and I've got to meet him at his flat. When I get the money off the mug I intend going home."

In the apartment the men drank whisky. Then something happened. Philip was "beaten and slashed into insensibility" in the bedroom. Roland ransacked the place, grabbed what he could before leaving Philip for dead amid "broken furniture and ornaments, torn from walls bespattered with blood".

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His own suit was so bloodied that he had to change into one from the wardrobe, dropping his diary in the process. The next morning, a maid found Philip. Roland was arrested a week later. At the Old Bailey he was convicted of robbery with violence and imprisoned for three years.

That night in August 1929, when two lives met, the sensational "Mystery of Mayfair" was born. Unfurling in the News of the World came the discovery of Philip's battered body and his desperate fight for survival, the nationwide hunt for his assailant, the courtroom proceedings and implicit sexual impropriety that captivated the reading public. Herein lies more than just a detective story about a brutal attack on an academic. For me, it raises another question - how do we write the history of homosexuality?

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In contemporary Britain, popular understanding of queer history is shaped by two assumptions. The first is that legal repression and social intolerance kept homosexuality firmly in the closet until "liberation" in the late Sixties. In this context, goes the argument, homosexual men were invisible and isolated, forced through desperation - like Philip - to seek partners in the most dangerous public spaces. The trial's conclusion supports this supposition. Roland pleaded "justification of the attack" - Philip "acted in such a way that, realising his intentions and being full of whisky, he became mad and hardly knew what happened". Set against his calculated theft, this defence rang hollow.

Still, there was a compelling assumption that Philip's motives for taking a stranger home could only be "indecent" - he'd got what he deserved. Judge Ernest Wild described Roland's action as "one of the gravest offences known to law, the maximum sentence for which was penal servitude for life".

Despite this, he passed "the least sentence commensurate with the offence".

Violent assaults on men like that could never be accepted unequivocally - but could be understood.

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Yet Wild's simmering hostility is only part of the story. The metaphor of the closet fails to capture queer life in the decade after the First World War. By following Philip and Roland, we enter an urban culture that was more visible than at any other time before the Seventies. Rather than being isolated, Philip moved through a city teeming with social and sexual opportunities. He wasn't the only queer man to rent an apartment in Half Moon Street; together with Hugh Walpole, Reggie de Veulle, Robbie Ross and Joe Ackerley he forged a remarkable enclave. Walking the streets around Piccadilly and Hyde Park, Philip entered an established public sexual milieu, where wealthy gentlemen, flamboyant "queans" and young working men met. Minutes away, a rich "scene" took hold in West End venues such as the Palladium's Circle Bar - Roland's regular haunt. This world held not only real perils, but alluring pleasures, too.

There is something strange here. Why was Roland drinking in bars and walking streets known as queer meeting places? Was he just looking for "mugs"? Such questions highlight the second popular assumption about queer history: sexual practices and identities have always been organised around the opposition of "homosexual" to "heterosexual" - you are either gay or straight. The task of the historian is thus to "recover" gay men's presence in the past. From this perspective, Philip embodies generations of heroic victims, Roland an oppressive straight world.

But Roland's character was more complex than this. True, he and the working men, guardsmen and down-and-outs he associated with robbed and blackmailed the wealthy men they met around Hyde Park. At the same time, they had sex with them and, on occasion, formed lasting relationships. Trading sex for money could be a solution to poverty. More importantly, it depended on working-class understandings of masculine "normality" that made relationships with women and men possible and pleasurable - as long as men maintained their manly dominance. In this world, manliness could mean fucking a man or beating him half to death. Roland was neither straight nor gay or bisexual - those categories cannot comprehend his actions. What was he looking for on Piccadilly? Profit, clearly; but also excitement, pleasure, sex, a chance to prove he was a man - on another night, that meeting might have turned out very differently.

So what do we make of the Mystery of Mayfair? The differences between Philip and Roland suggest the complexity and diversity of queer life between the wars. More disturbing, their meeting forces us to recognise the difference - the queerness - of the past. Philip and Roland did not inhabit a "gay" world as we would understand it. The task of finding "our" hidden past fails to appreciate the historically specific ways that they understood their own lives. Making sense of this meeting might also prompt us to consider how we write the history of 20th-century Britain itself. Too often, queer history and its subjects have been ignored as marginal or irrelevant. But it is impossible to comprehend Philip and Roland's lives outside more established historical themes - the Depression or the nature of the modern city. In exploring their lives, our understanding of these themes is enriched. We should write men such as Philip and Roland back into Britain's history and, in the process, rewrite that history.

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Above all, the Mystery of Mayfair dramatises the troubling paradox of exploring the queer past. Think about how these two lives have entered the historical record. Until recently, a battery of laws exposed nearly every aspect of everyday queer life to the risk of arrest. Philip - and men such as him - worked hard to avoid attention. He moved discreetly across London, leaving neither visual nor written traces of his behaviour. As a result, the legal records and newspaper reports produced when men entered the dock or witness box are one of the few ways through which we can write homosexuality's history. Without this trial, these ordinary lives would remain invisible. It is possible to read such fragmentary sources, as I've done here, to suggest how men successfully created a vibrant queer world at London's heart - but you can never escape the fact that entering the public record was a disaster in their lives. As with so many others, Philip's story is preserved for us because he momentarily failed to negotiate the dangers inherent to queer life. We build a queer history on countless individual tragedies.

Matt Houlbrook is lecturer in 20th-century British history at Liverpool University. His book Queer London: Perils and Pleasures in the Sexual Metropolis, 1918-57 is published in September by the University of Chicago Press, ?20.50.

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