Martin Murphy is a distin-guished historian, for many years a research associate of the Oxford Dictionary of National Biography (though no one could wear great learning more lightly).

More to the point, he is a passionate lover of all things Spanish – he taught for a time at Seville University – which explains his new book, The Duchess of Rio Tinto – well, the second half, anyway. The book – an often hilarious read of 120 pages – follows the lives of two 18th-century adventurers/gamblers/chancers – I’m not sure there is an exact word – Lady Mary Herbert and her passionate but constantly thwarted admirer Joseph Gage.

It starts with them immersed in the world of Paris high society around 1720, when the Regent – the Duke of Orleans – had been unwise enough to allow a young Scots financial whizz-kid, John Law, to float a South Sea Bubble-like scheme based on new French territory in Louisiana. The collapse of Law’s “System” in 1721 (brilliantly described) left both Lady Mary and Gage not only owing thousands, but immersed in “a flood of litigation which would last the rest of their lives”.

Their decision to recoup their fortunes by reopening ancient mines in the Spanish Sierra, north-west of Seville, is to be explained by Mary’s family, the Herberts, having mining in their blood since the discovery of iron ore on their Welsh estates in the 17th century.

But Murphy’s researches reveal an incredible story, in which – despite a low point where rival gangs of miners knock each other senseless in a tavern in Cadiz – Gage successfully opens one of the mines, and Mary gains royal approval. Touchingly underlying this whole tale, however, is Jacobitism – the ultimate lost cause – in which Lady Mary remained a believer to the last. She died in 1775, triumphantly happy to have been addressed at last as “Duchess” by the Old Pretender – for her, the legitimate king – James III.

The Duchess of Rio Tinto, Martin Murphy, St Clements Press, £12.99