To mark the launch of her new book, The Casual Vacancy, Harry Potter author JK Rowling held a question and answer session in London's Southbank Centre with 900 eager fans.

Rowling became the world's first billionaire author following the success of the Harry Potter franchise. She famously came up with the idea for Potter while travelling on a train, she revealed The Casual Vacancy came to her in similar circumstances.

She said: "I had the idea for The Casual Vacancy while I was on tour in America for Deathly Hallows, and I had the idea on a plane. Yeah I know, something about me and vehicles, I have to be moving, clearly, to have an idea."

The gritty tale about a small English town has received mixed reviews from critics, Rowling was quick to defend her work.

She said: "It is a humorous book. Some of the humour may be rather dark in places, but yes, that's life in a small town."

The author also revealed her future plans, claiming to have two books for children that are "pretty well developed", adding she knows what her next tale for adults will be, although it is "not very well advanced".

The Casual Vacancy is on sale now.