When Sylvia Plath's marriage to Ted Hughes foundered in August 1962, her family assumed that she would move herself and her children back to America. ‘The worst difficulty is that Ted is at the peak of his fame,’ she wrote to her mother on 21 October, ‘and all his friends are the ones who employ me.’ Aurelia Plath published her daughter’s Letters Home in 1975. ‘I opened a joint account in a London bank,’ she wrote in a note, ‘so she could use it in any emergency, hoping she would consider returning to the United States. We, as a family, were prepared to set her up in her own apartment here.’

But Plath stayed in England. She offered her mother many reasons why she didn't want to leave: she would feel she was running away from her problems; it would be too disruptive for the children; Hughes would be more likely to pay child support if he saw his son and daughter regularly; she was getting work through the BBC and The Bell Jar would soon be released by a British publisher. But above and beyond all these justifications, there was the National Health Service, and the knowledge that she would have no health insurance if she returned to America.

‘I need help very much just now,’ she wrote to her mother on 16 October. America ‘is impossible. I can go nowhere with the children, and I am ill … I have free doctor's care here.’ In a letter to her brother Warren two days later, she said:

I am a writer and that is all I want to do. Over here I can earn quite a bit from the radio, live on little, get free medical care, and have had my first novel accepted (this is a secret; it is a pot-boiler and no one must read it!) and am ready to finish a second.

When Plath moved from Devon back to London in the late autumn, one of the things that seemed to thrill her most was the easy access to medical care. ‘I am back with my panel of blessed, excellent doctors,’ she told her mother in November. A month later, she wrote: ‘It is such a relief to be back with my wonderful and understanding Doctor Horder.’

Plath had reason to be wary of the American healthcare system. During the breakdown that led to her first suicide attempt in August 1953, when she was a student at Smith College, one of the factors that contributed to her decision to kill herself was the knowledge that her family would not be able to afford the medical care she needed. She had already been given electroshock therapy by a psychiatrist at a private hospital, and felt guilty about the expense. The therapy was administered improperly and painfully, and made Plath – who had no idea that the treatment had been botched – feel worse than ever.

After her suicide attempt, Plath had a brief but traumatic experience in the psychiatric ward of a public hospital; Olive Higgins Prouty, the writer and philanthropist who funded Plath's scholarship to Smith, then paid for her to go to McLean Hospital, a private psychiatric hospital in Belmont, Massachusetts, where over four months she underwent both insulin shock therapy and electroshock therapy. She returned to Smith in January 1954, certain that without her time at McLean she would never have been well enough to return to Smith so soon, if ever.

These experiences must have been on Plath's mind when, shortly after moving back to London at the end of 1962, her depression began to worsen. Her situation was dire; she was a single mother caring for two toddlers, rising every day at 4 a.m. to write – her only source of income – before the children woke up. That winter was the worst to hit London for more than a century. ‘The lights failed and candles, of course, were unobtainable,’ Al Alvarez wrote of that winter in The Savage God (1971). ‘Nerves failed and marriages crumbled. Finally, the heart failed. It seemed the cold would never end.’ Plath's children endured several bouts of coughs and colds, and then in January 1963 all three of them came down with a bad case of the flu. The Bell Jar came out and received little attention; what few reviews it did get were lukewarm at best. Life in London, which just a few months ago had looked so promising, began to seem impossible and hopeless.

Plath recognised how dangerous her mental state was and sought help. Her doctor came to her flat every day; she started taking anti-depressants; when Plath felt especially unable to cope, she and the children went briefly to stay with friends. Her doctor was trying frantically to get her a hospital bed, but none were available. (Still an all too familiar story.) ‘I could never afford to live in America,’ she wrote in her last letter to her mother. ‘I get the best of doctors' care here perfectly free, and with the children this is a great blessing … I shall simply have to fight it out on my own over here.’ The letter is dated 4 February, a week before Plath's suicide. At the end of it, she wrote:

I am going to start seeing a woman doctor, free on my the National Health, to whom I've been referred by my very good local doctor, which should help me weather this difficult time. Give my love to all.

On 12 February, Aurelia Plath's sister received a telegram from Hughes: ‘Sylvia died yesterday.’

Some of the factors that contributed to Plath's suicide have been endlessly picked apart, especially her mental illness and the collapse of her marriage, but neither of those tell the full story. Her story is also the story of a single mother who could not afford healthcare in the country where she was born, and died, at least in part, because of that.