Episode seven focuses on the Queen’s two “hidden” cousins, Katherine and Nerissa Bowes-Lyon, who were born with severe learning difficulties and admitted to a mental hospital in 1941. They were the daughters of John Herbert Bowes-Lyon – the brother of the Queen Mother – and his wife, Fenella.

Infuriatingly, because of the deep stigma attached with mental health in that era, the sisters were listed in the 1963 edition of Burke’s Peerage – a reference book to the nation’s aristocracy – as having died in 1940, when in fact they were both still alive.

They secretly continued to live most of their lives in Royal Earlswood Hospital. Nerissa died aged 66 in 1986 and Katherine died aged 87 in 2014.

The Crown season 4: the heartbreaking truth behind this unexpected episode