Amid the sprawling mass of complicated statistics and medical evidence deployed by the prosecution against Lucy Letby there was one devastatingly simple piece that was viewed as little short of a smoking gun.

A pale green Post-it note – one of a cluster of many messy handwritten jottings found among the young nurse’s belongings at her home. On it had been written, in large, emphatic capitals, the words: ‘I AM EVIL. I DID THIS.’