Like old treasure maps (“Yo-ho-ho and a bottle of rum!”) there is something fascinating about dead men’s manuscripts, whether they turn up in attics, junk shops or the safekeeping of friends. I thought this a while ago when such a manuscript came into my hands, written by man named Tom Cullen about a man named Gerald Hamilton – at which point I should probably interrupt my story and tell you who Gerald Hamilton was.

He was either “a sweet, darling, loveable man”, in the words of a friend, or he was “one of the most evil men alive today”, in the words of a Sunday newspaper which ran a five-part exposé of him in 1966 as “the wickedest man in the world”, written by the novelist Robin Maugham. “Gerald Hamilton”, ran the first headline: “Traitor, Con-Man and Crook (Even his Name is a Fake)”. Away from the world of yesterday’s tabloids, Hamilton is remembered as the real-life model for the seedy but beguiling Mr Norris in Christopher Isherwood’s 1935 novel Mr Norris Changes Trains.

I’ve loved the Norris novel ever since reading it in bed with a hangover – someone said it would make me feel better, and it did – and been fascinated by Hamilton himself ever since reading Conversations With Gerald, a small masterpiece by John Symonds (a wonderful writer in his own right, who deserves to be remembered for more than his classic Crowley biography, The Great Beast). Symonds’s Gerald comes over as eccentrically opinionated but highly endearing, which wasn’t how everyone saw him: the writer and Fitzrovia character Julian Maclaren-Ross had an unsubstantiated story about a young man who was stabbed to death after taking Gerald’s side in an argument. The bereaved parents complained to Hamilton, only to be brushed off with “Goodness gracious me! Young men today seem to want to live forever!”

Sticking closer to the facts, Hamilton was interned in both world wars as a threat to British security, and he certainly knew a lot of people, from Guy Burgess to Aleister Crowley (he shared a flat with Crowley in Berlin, and Crowley received money from the Special Branch for reporting on him – in fact it was rumoured that they spied on each other and the flat paid for itself). And whatever life threw at him, Hamilton never lost his impeccable Edwardian manners, his love of wine and food, or his unflappably camp charm: “We live in stirring times,” he liked to say, “tea-stirring times.”

I was researching Hamilton for The Dictionary of National Biography when I heard rumours of a completed life somewhere, already written but unpublished. My friend Tim d’Arch Smith vaguely remembered one, and the late Peter Burton (a friend of Hamilton who became Rod Stewart’s publicist) had read it thirty years ago and knew who it was by. It was by Tom Cullen, whose work I already knew and admired, having read his biography of Maundy Gregory. So what had happened?

It seemed Cullen’s book had been blocked for legal reasons, several people objecting to the contents, and in particular Hamilton’s friend Robin Maugham. Burton knew what he was talking about here, having been Maugham’s secretary. Maugham claimed to object to a story that he knew Hamilton’s death was faked, and that Hamilton had been seen riding round Sloane Square in a taxi a week later – which would have been totally in character – but the real reason was more likely to be a clear inuendo that Hamilton procured young men for him, and that the two of them had been in cahoots on the flagrantly insincere Sunday People potboiler about Hamilton’s wicked life.

Cullen – an American expat who lived alone in North London – was dead, but the wills and probate office in Holborn is a terrific resource for a researcher: as often as not it will bring up the dead person’s family and friends, complete with addresses. And so it was that I traced some of Cullen’s friends, one of whom directed me to another friend and executor named Reed Searle, now returned to the United States. Reed Searle, it turned out, had rescued the Hamilton manuscript from a more general bonfire of Cullen’s papers, and more than that he was willing to let me see it for my DNB research. Now we were getting somewhere.

Like a lot of people who write, I find the idea of unpublished work emotive: a friend of mine was murdered on the night after he had finished his first book, and one of the last things he saw must have been the theft of his computer with the book in it, while my own last thought during a serious car crash was not that I was going to die but that I was never going to publish my book on Samuel Beckett. As I read the manuscript I felt for Cullen, and felt that his book deserved to exist in the world.

Happily Reed Searle felt the same way, excellent man, and we were in business. I began the long slog of scanning it (it was an exceptionally messy manuscript, covered throughout in hand emendations and comments from at least two people, which threw the scanner and produced something like algebra) before some editing and re-chaptering. Cullen’s working title – Mr Norris Changes Names – wasn’t quite satisfying either, and seemed to belong to a period (circa 1980, when the whole Isherwood-Berlin-Cabaret thing was much bigger) when Norris was more familiar and could be joked about.

But what to call it? “The Importance of Being Gerald” was one idea, and “Charm Is A Mystery” was another, from the W.H.Auden poem where he addresses Hamilton with “Uncle Gerald, your charm is a mystery”. Hamilton was a charmer but skulduggery, fraud and treachery seem to have been second nature to him, so “Chateau-Bottled: The Reprehensible Life of Gerald Hamilton” was also a possibility, after Anthony Powell’s description of someone as a “chateau-bottled shit” – but perhaps it was too obscure.

The Man Who Was Norris, as it’s finally called, is probably the best picture of Hamilton we shall ever have, particularly because Tom Cullen was able to speak to so many people who had actually known him. Along the way it sheds some vivid sidelights on the social, sexual and political history of his times. Cullen’s Hamilton is perhaps less endearing and more shocking than the Hamilton of Conversations with Gerald, but the jury are still out on just how bad he really was. Readers will have to decide for themselves. Seen from one angle, a man who can defraud the Save the Children Fund while running guns for the IRA and professing pacifism is something more than just a loveable rogue – and yet, as Mr Norris liked to say, “There are some incidents in my career, as you doubtless know, which are very easily capable of misinterpretation.”

For my part, I’m extremely happy to have rescued it for posterity. It is not quite in the same league as pulling a child from a freezing river (I know, I know) but if the Recording Angel ever has to balance my affairs, I hope he will remember that I once saved another man’s manuscript.