Just over a year ago I found myself standing in the drawing room of a ground-floor flat in Cadogan Place, Belgravia. Here, strewn all over the carpet, in crates, packing cases and capacious box files, were the possessions of Janetta Parladé, who had died a few days before at the age of ninety-six. Parladé had several claims to fame, but literary history will probably remember her as the last of the ‘Lost Girls’, as defined by Peter Quennell in his memoir The Wanton Chase (1980): rackety, well-bred and emotionally damaged young women who hung around the fringes of literary London in the 1940s, worked on Cyril Connolly’s monthly magazine Horizon, had affairs with everyone from Lucian Freud to Arthur Koestler and were inexorably drawn into the web of myth that Connolly projected around his own complex romantic life.

While Janetta tended to play second fiddle to better-known convives such as Barbara Skelton and Sonia Brownell (soon to become the second Mrs Orwell), this isn’t to diminish her importance to Connolly and his circle. She was, for example, present on the occasion when the the word ‘bugger’ (in a short story by Julian Maclaren-Ross) had to be struck out by hand from six thousand copies of a Horizon number by the magazine’s staff after the printers objected to it. A horrified Evelyn Waugh, to whom she once opened the door without having put on her shoes, christened her ‘Mrs Bluefeet’ and gave her a minor role in Unconditional Surrender (1961). As Sonia’s bosom companion, she visited Orwell as he lay dying at University College Hospital and was a witness at their wedding.

A small matter of seventy years later, courtesy of two of her daughters, there I was, present at what turned out to be the literary equivalent of an unexpected banquet. Much to everyone’s surprise, Janetta turned out to have kept countless mementos from her long and colourful life. ‘Is there anything you’d like to borrow?’ somebody asked. I escaped into the Belgravia sunshine with a file of Connolly’s letters, a folder full of correspondence from the mid-1940s, an eye-popping late 1970s lament from Sonia in which she complains of being swindled by Orwell’s accountants, and a suspicion that Lost Girls, the book I had just finished writing about this tantalising part of the Blitz-era demographic, might need a radical rethink.

Historians sometimes talk about ‘Bentley’s luck’. Richard Bentley (1662–1742) was a celebrated classical scholar and master of Trinity College, Cambridge, who fell out with his colleagues and would probably have got the sack had the bishop commissioned to investigate the case not dropped dead the night before the inquiry. The morning spent in Janetta’s flat was, I realised, an example of ‘biographer’s luck’, which might be defined as encountering some vital new source just before the manuscript is about to disappear into the production process.

Some examples of biographer’s luck? Meeting a friendly art dealer with a hitherto unknown scrapbook that Thackeray had compiled in the 1830s. Sitting by a fax machine at three o’clock in the morning as it began to pump out several letters that Orwell had written to Malcolm Muggeridge, run to earth at an obscure Midwestern Bible college. Being granted access to the diaries and letters of Arthur and Dorothea Ponsonby – parents of the notorious Elizabeth – in which the activities of the Bright Young People are remorselessly laid bare and the echt-Victorian Dorothea wonders how on earth anyone could find Vile Bodies amusing.

Naturally, there are times when biographer’s luck runs out. The Pathé footage of the Eton wall game from 1921 that contains Orwell’s only appearance on film turned up three months after Orwell: The Life came out. And I felt a pang of solidarity for the audience at a Laurie Lee memorial evening at Gloucester Town Hall some years ago who were informed that the organisers had arranged a special treat. At a particular point in the proceedings the lights would be dimmed, the mayor of Gloucester explained. When they came back on, the original ‘Rosie’ of Cider with Rosie would declare herself. Amid an expectant hush, the lights flicked off and then on again, whereupon it was revealed that no fewer than four elderly ladies had clambered eagerly to their feet.

Where do biographers get their inspiration? Novelists are usually happy to admit to a sudden flash of insight (Orwell got the idea for Animal Farm by watching a small boy goad a carthorse along a country lane), but for life writers it is more often a case of gradual immersion. You are interested in a particular subject; basic research hardens this interest into an obsession, after which the subject becomes an unshiftable part of your mental furniture. Curiously, Lost Girls developed out of a single incident.Talking to the daughter of a Lost Girl in her north Norfolk kitchen, impressed by the number of appearances her enigmatic mama had racked up in the literary autobiographies of the postwar era, I made a rather naive remark about the glamour of being born into a world where the man snoozing in the deckchair at the bottom of the garden might turn out to be E M Forster.

My friend rose up out of her chair and loomed over me. ‘You have no idea,’ she said, ‘quite how awful my childhood was.’ Suddenly, the ghosts of Forster, Connolly and Waugh receded and I was left with the vision of a small, terrified child. I later discovered that she had been sent to a children’s home at the age of two for fear of what her stepfather might do to her.

With biographer’s luck comes biographer’s responsibility. The Lost Girls may all be safely dead, but many of their descendants are very much alive. The materials from which this communal life was reconstructed are sometimes startlingly intimate. In particular, the letters that Connolly’s ex-girlfriends Lys Lubbock and Diana Witherby wrote to him in the aftermath of his crack-up make uneasy reading. The girls know Connolly has behaved badly to them. They know he has exploited their good natures and serially betrayed them. And yet the lure of his personality makes them desperately regretful, well-nigh drowning in remorse and self-abnegation.

How would Lys and Diana have felt about the details of their relationships with Connolly being made public? When dealing with a literary decade dominated by men, there is an argument for giving the women who typed their letters and went to bed with them a voice. Janetta Parladé turned out to have left behind an unpublished memoir that contains, among other highlights, an eyewitness account of the Orwell wedding. ‘She wanted this to be read,’ one of her daughters explained. That’s good enough for me.