But her full-time teaching career was cut short by debilitating hemiplegic migraines, which resulted in severe pain and paralysis down one side of her body.

Instead, she took up tutoring and - unknown to her family until after her death - began writing books. She attempted to control her pain with medication prescribed by doctors, including the strong painkiller tramadol.

Pathologist Jonathan Pearson told the inquest that the condition of Miss Gradwell's body made finding a cause of death difficult, but there were no signs that she had been assaulted by another person.

However, he added that toxicology tests revealed high levels of tramadol in her body.

"It is the only evidence we have of something abnormal that could explain the sudden death," said Mr Pearson. "I accept entirely it is not conclusive, but on the balance of probabilities it is the best evidence we have to explain the death."

Miss Gradwell had been dressed in her pyjamas at the time of her death and it is thought she had been sleeping on the sofa as there was a quilt and pillow.

Empty packets of tramadol capsules were found and it was suggested that Miss Gradwell may have taken too much of the drug in a bid to relieve her migraine pain.