“The thing about Roald was that he loved children, and wrote for them,” Roald Dahl’s widow Liccy says. “He had the mind of a child, which was why he was able to reach them. Once you’d read one chapter you longed for the next. So many children’s books then were boring for adults to read, but he always made you long to go to bed. He said that in writing for children you had to grasp them by the neck in the first sentence. He had a perfect simplicity in the construction of each sentence which is what made him so readable.”

Their marriage, which lasted until Dahl’s death, coincided with his most fertile time as a writer, in which his retreats to his famous writing hut produced The BFG, and The Witches; but Charlie and the Chocolate Factory came before they met, at a tragic time in Dahl’s life.

Dedicated to Theo, the son who was seriously injured as a baby, it was written at a time when Dahl was doing all the cooking, cleaning and childcare for four children while his first wife, the actress Patricia Neal, was in a wheelchair. Although the newly opened Roald Dahl Museum in Buckinghamshire contains the three notebooks in which he wrote down his ideas for stories, the inspiration for his most famous book is never mentioned.

According to Liz Whittingham, collections manager at the museum, it probably came from his memories of his time at Repton school in the 1930s, when Cadbury would send the boys a box of new chocolate lines to taste every year. (Sadly, Cadbury refused to have anything to do with sponsoring the museum.) Unable to use a typewriter, Dahl wrote in longhand, and was a meticulous editor of his own work, which is why the “spotty chapter” published today was excised. Originally there were to have been ten children touring Willie Wonka’s factory, but these were whittled down to five. Miranda was the last to go, being “too similar to Veruca Salt”, according to Whittingham. The book took two years to write, and seems to have been started around the time his daughter Olivia died of measles in 1962.

It is no secret that Dahl hated the first film of Charlie and the Chocolate Factory, which ruined not only his savagely funny songs but starred Gene Wilder, who had no satirical edge. Dahl wanted Spike Milligan to play Wonka, but the producer turned him down, and “broke his heart”. The new film by Tim Burton, starring Johnny Depp, is a “very, very good partnership”, according to his widow.

The author’s success made him money only towards the very end of his life, in the late 1980s, and was drained by the medical problems of his family as much as by the fact that nobody wanted to publish his first book for children, James and the Giant Peach. He went 50-50 with the one publisher who did. He was never confident that he would be able to write a new book, telling Liccy that “If you’re confident you don’t do good work”.

Dahl felt strongly that fame in your own lifetime bred mediocrity, and felt shunned by the establishment, who did not feel that children’s writing should be celebrated. But he did say: “If I knocked on any door anywhere in the world where there was a child, I’d be offered a cup of tea.”

Charlie and the Chocolate Factory is showing in cinemas from Friday

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