The next day, they drove to the studio, but Leigh was far too deranged to work – “Eyes overbright, she chattered ceaselessly.” One night, her maid rang Niven and begged him to come because her mistress was “possessed”. He found Leigh standing at the top of the stairs, naked. “Her hair was hanging down in straggly clumps; the mascara and make-up made a ghastly streaked mask down to her chin; one false eyelash was missing; her eyes were staring and wild.” He tried to put his arm round her, but she fought back. “She was spitting like a panther, biting, clawing and kicking,” Niven wrote in his memoirs. Eventually, he managed to pinion her arms and call for a doctor, who came with two assistants and a large syringe. “I found I had come to hate her,” wrote Niven.

Olivier had not quite come to hate her, but by now he was exhausted. ECT treatment calmed her down, but, “She was no longer the person I had loved,” and he felt like a hollow man. He conveyed that hollowness most brilliantly in The Entertainer at the Royal Court, where he fell deeply in love with Joan Plowright, and asked Leigh for a divorce. She begged him to stay, but he wouldn’t: he had had enough.

As a former executive editor of The Hollywood Reporter, Galloway is inclined to hyperbole, but he ­certainly knows his way around the film industry, and he has ­conducted valuable interviews with people who knew the Oliviers, such as Hayley Mills, and the actress Sarah Miles, who had an affair with Olivier in 1962 when she was only 21, and then saw him again towards the end of his life, when he was suffering a terrible skin disease that meant that, although they lay in bed together, he could not touch her.

By 1958, Leigh had taken up with a kind and handsome actor called Jack Merivale, but she always kept talking about Larry. In May 1967, she started coughing. She had been diagnosed with tuberculosis years earlier, and now it returned. She refused to go to hospital. On July 8, Merivale found her lying on her bedroom floor, dead. She was only 53.

Merivale broke the news to Olivier, who came round at once. “I stood and prayed for forgiveness, for all the evils that had sprung up between us.”

Galloway calls this “the Romance of the Century”, but it sounds more like a tragedy to me.

Truly Madly by Stephen Galloway is published by Sphere at £25. To order your copy for £19.99, call 0844 871 1514 or visit Telegraph Books