For thirty-nine years, Patricia Neal's brain had successfully coped with about 750 millilitres of blood pumped up through her cerebral arteries every minute of every day. But on 17 February 1965 things went disastrously wrong. Neal had won the Oscar, the Academy Award for Best Actress in Hud, the year before, and was back in Hollywood working on her next movie project, Seven Women. She was three months’ pregnant with her fifth child, Lucy, and was at home giving seven-year-old Tessa a bath when suddenly a pain shot through her head. Initially she wondered if she had just overdone it at work. It had been a long day. Filming had only just started, and she had spent most of the day on the back of a donkey. Dahl came upstairs with a Martini for Pat to find that she was staggering around the bedroom, clutching her head.

‘I've got the most awful pain,’ she told him. ‘I think there's something wrong.’ She was swaying and wincing in agony, pressing the palm of her hand against her left temple. ‘I've been seeing things too.’

‘What sort of things?’

‘I don't know. I can't remember.’ She lay on the bed. ‘The pain is terrible.’

‘Is it only in one place?’

‘Yes. Right here.’

In his bed at the John Radcliffe Hospital, Dahl sat up, showing me exactly where she had pointed – the left temporal bone. I'd had a busy day, and was glad to settle into the chair by his bed and listen; the soft folds of Dahl's large dressing gown, draped across the chair, had a faint sweet smell of pipe tobacco, and I wondered what Dahl had been up to in the day.

‘The pain was here.’ He gestured again, his face alert. ‘It happened in an instant. Suddenly Pat's head jerked back and she lost consciousness. I just knew she was in trouble. She looked like she was dying right in front of me.’ Although he had been trembling with fear, Dahl didn't panic. Instead, his calm, calculating, problem-solving mind took over, the one that had seen him coolly extricate himself from a crashed plane as it exploded around him, the one that had methodically created a new device to solve the problem of hydrocephalus, the one that had systematically reviewed publications looking for a link between severe measles and smallpox vaccination.