Not a child, which was short-sighted of me.

A librarian. Or a racing driver for Ferrari F1.

Archaeologist, pop star, store cashier.

I just thought I was going to go into graphics and painting, which is what I wanted to do.

So given a Time Turner, I would tell my 21-year-old self that personal happiness lies in knowing that life is not a check-list of acquisition or achievement. Your qualifications, your CV, are not your life, though you will meet many people of my age and older who confuse the two. Life is difficult, and complicated, and beyond anyone’s total control, and the humility to know that will enable you to survive its vicissitudes.

Moments of pure inspiration are glorious, but most of a writer’s life is, to adapt the old cliché, about perspiration rather than inspiration. Sometimes you have to write even when the muse isn’t cooperating.

I try to start work before 9am. My writing room is probably my favourite place in the world. It’s in the garden, about a minute’s walk from the house.

As the screenwriter, the majority of my input comes at an earlier stage.

So why do I talk about the benefits of failure? Simply because failure meant a stripping away of the inessential. I stopped pretending to myself that I was anything other than what I was, and began to direct all my energy into finishing the only work that mattered to me. Had I really succeeded at anything else, I might never have found the determination to succeed in the one arena I believed I truly belonged. I was set free, because my greatest fear had been realised, and I was still alive, and I still had a daughter whom I adored, and I had an old typewriter and a big idea. And so rock bottom became the solid foundation on which I rebuilt my life.

I can remember feeling elated after writing the first Quidditch match, which flowed out of the pen and was barely revised afterwards.