Lunacy, as lovers of Wilkie Collins’s novels will be aware, was a lucrative business in 19th-century England (and not only for the owners of the country’s innumerable asylums). Anybody who wanted to lay their hands on a fortune – as did Collins’s magnificently wicked Count Fosco and his less lovable sidekick, Sir Percival Glyde – found it remarkably easy to lock away a perfectly sane heiress and to seize her estate.

The case of John Charles Wallop, third Earl of Portsmouth, newly investigated and beautifully narrated here by Elizabeth Foyster, a historian specialising in family history, was complicated by the fact that the earl’s eccentric habits offered him a wobbly perch on the Humpty-Dumpty wall of lunacy. Portsmouth was