'It was never my intention to publish'

E L James explains how she came to write 50 Shades of Grey, the 'mummy porn' that became the best-selling book in British history. By Adam Sherwin

The queue of models and hipsters snakes outside the Shoreditch House club, demanding access to the exclusive roof-top event. The star of the show, meanwhile, is keeping everyone waiting in her dressing room.

Finally E L James sweeps into the room for a rare speaking appearance which marks the 50 Shades Of Grey author's formal elevation from self-published fan writer to literature's Lady Gaga. Listed in Time magazine's 100 Most Influential Women, the first-time writer whose erotic fantasy novels have become a 40-million-selling phenomenon, is adapting fast to diva-dom.

James, 48, who is from west London and has two children, agreed to a brief on-stage interview, held at the east London members' club, to launch the latest brand extension she has endorsed.

50 Shades Of Grey: The Classical Album, released by EMI, features 15 pieces of music referenced during the steamy, S&M-tinged encounters between the student Anastasia and billionaire Christian Grey.

Such is James's influence that a mere mention in the first novel of Spem in Alium, a 16th-century motet for 40 voices by Thomas Tallis, sent the download to the top of the classical charts.

Mark Ellen, the music journalist who has interviewed mega-stars from Michael Jackson down, was selected to conduct the conversation in front of an invited audience, secured in risqué handcuffs after gaining entry.

Ellen informs the audience that James will speak for only 15 minutes and will not take questions or sign books, restrictions no less rigorous than those imposed by the King of Pop's handlers.

When James, whisked through the throng by a retinue of handlers, takes the stage, she parcels out insights into the extraordinary sequence of events that has seen the former television executive earn an estimated £20m inside 18 months.

"I thought I'd continue working and writing in the evening like I was doing. But it's just taken off and I still don't quite believe it," she says.

A mid-life crisis spawned the best-selling book in Britain of all time. "I worked for a company and I was incredibly miserable. Then I discovered fan fiction after reading Twilight. I started writing. It was good. I stopped going out and getting drunk so much."

When 50 Shades was released as an e-book last year, James was taken aback by the demand. "It was never my intention to publish; I really did it for myself. Then a friend of mine had his book published and I thought 'if you can do it, I can have a go'. So I did and here I am."

Yet James is also a smart marketer who ensured that her target female audience could indulge in 50 Shades' bondage fantasies without public embarrassment. "I designed the book covers," she said. "I used to read a lot of romantic fiction on the Tube and they had these horrible covers of women with their clothes falling off. So I designed really discreet covers for the books so no one would ever know this was a very salacious novel."

It's sex though, the kinkier the better, that the audience, which includes the actress Jaime Winstone and refugees from the cast of Made In Chelsea, really wants to hear about. "I had a lot of fun researching these books," James says. "Music to f*** to. We all want that. I have songs I write sex to. The Black Eyed Peas' 'Sexy' is a very sexy song."

Defending her books as "passionate love stories" – against claims of "mummy porn" – the writer argues that they have changed lives for the better. "People of all ages are reading it, from girls at university to women in their 90s. Those emails are an interesting read."