Jill Murphy is on the phone, telling me about the last time she cried. “I was with my son. We’ve had to face my illness, which at one point neither of us could do, because we had always taken each other utterly for granted. We are very close and we were trying not to even admit quite how ill I had become, because neither of us wanted to upset the other or face what it meant to us.”

This was at the end of last year, and Murphy – the 72-year-old author of The Worst Witch, who died of cancer last Wednesday – was talking to me for an interview in Observer Magazine. “In the end, we got outside help, mainly from friends, and we talked about my illness and how we were feeling. And, of course, we both cried, and felt much better for it.”

It was one of the last interviews she ever gave, and until now, in accordance with her wishes, has never been published. At the time we spoke, the venerated children’s author was feeling hopeful about her chances of containing the aggressive secondary bone cancer that was spreading around her body, following five years of remission from breast cancer. However, after she received bad news about her prognosis, her publicist contacted the Observer and explained that Murphy no longer wanted to go public about her illness and was about to undergo intensive chemotherapy.

When we spoke last September, Murphy said: “What I miss at the moment is drawing and writing. I hadn’t realised, until I got ill, that I draw and write constantly. Normally, I’ll cover the bottom of a tissue box or a shopping list with drawings from the next book I’m working on. Now, my hands are damaged so I can’t.” But her desire to write and draw was just as strong as ever. “I’ve got some drawings for my next book in a tin on my desk, and, as usual, they are calling to me every day, just as they always have done.”

Murphy’s career as an author and illustrator began when she was still at school. She was just 18 when she completed the manuscript for The Worst Witch and created the most accident-prone student witch in history, Mildred Hubble. The book sold out within two months of its publication in 1974 and has become an international bestseller – never out of print and adapted often for stage and screen.

“People assume my greatest achievement is The Worst Witch. But it isn’t. It’s my son Charlie. He tops everything for me,” Murphy told me.

But she is obviously extremely proud of Mildred and her enduring appeal. “Children often say The Worst Witch was the first book they actually read by themselves. And occasionally I get a letter saying this was my favourite book as a child and now my grandchild likes it. I couldn’t ask for more, really, than a legacy like that, which will go on. I feel so grateful.”

Although The Worst Witch is a novel, Murphy has also won countless awards for her picture books, especially about the Large family of elephants, such as Five Minutes’ Peace and All in One Piece. Her most recent work, Just One of Those Days, starred the bears last seen in her very first illustrated title, Peace At Last. It was published last September and she told me she had completed it just three weeks before she started to get ill. “It was almost like magic. Like somewhere inside, I knew this was going to happen.”

She worked more intensively on that book than any she had ever created before, she said. “I did seven months of being up virtually all night and all day. And when I finished it, I was 100% thrilled with it.” This was unusual, she said, as she is normally very critical of her work. “Usually, I haven’t done it perfectly, but this book was different … I couldn’t have done it any better.”

She reminisced about her own childhood, growing up in postwar Britain. “I had a classic 1950s childhood. My mum was at home, because in those days that’s what mums did. My dad worked in an aircraft factory. He was up really early in the morning, came home exhausted at night and I didn’t really see him.”

At the age of six she was already writing stories. “I made my own library by stapling the pages of my books together.” She was always a talented writer and artist but “bumped along at the bottom” at grammar school: “The whole ghastly experience gave me the Worst Witch, because I was Mildred, one of the worst students at the school.”

Later, she was chucked out of art school. “But a tutor there gave me the best advice I have ever been given, and that was to always leave your working desk ready for the next morning – even if you go to bed at 6am, which I often did. Beforehand, I’d sharpen all my coloured pencils, clean the desk surface and put everything back, so that my lovely desk would attract me back to work later.”

Being so ill, she admitted she had asked herself questions such as, “Why has this happened to me?” But when she looks back, “I feel very lucky that I have no regrets at all. Before this happened, I was very happy with life”.

She said she certainly wasn’t planning to start complaining at the end of a long life lived “surrounded by love”. “I have loved every minute of getting older. Since getting ill, I’ve realised what an amazing time I’ve had in just about every department and been taking it all for granted. And if you have had that – and you’ve got wonderful relatives and friends supporting you – then you never want to leave the party. Ever.”