A British merchant ship called the Westmorland, en route from the Italian port of Leghorn to England, was on 7 January 1779 engaged off the coast of Spain by two warships from France, which had just entered the American War of Independence. We do not know exactly what happened – probably terrifying bursts of cannon fire were exchanged, for the Westmorland was armed – but the British ship was captured and taken to Málaga.
The Westmorland's cargo consisted largely of Italian foodstuffs: olive oil, anchovies and Parmesan. But there were also 54 crates of paintings, engravings, antique objects, marble sculptures, furniture and books. It was a precious cargo being sent back home by a number of aristocratic British gentlemen on the 18th-century equivalent of an elite gap year, the Grand Tour.
According to the director of Oxford's Ashmolean Museum, Christopher Brown, they "had been to all the grand buildings and churches and brothels in Europe – and had then gone shopping in Rome".
In Málaga, the anchovies, oil and Parmesan were sold off. The French captain sent one of the paintings – Anton Raphael Mengs' Liberation of Andromeda by Perseus – to the naval minister in Paris, who sold it on to Catherine the Great. (Mengs was the most famous artist of his day – equivalent to Damien Hirst, according to the Ashmolean's senior assistant keeper Catherine Whistler.) But the remaining 54 crates – containing about 743 objects – languished in a warehouse for four years, until the Spanish king acquired the lot.
And so the story might have ended: just another ship's cargo gone, the acute regret of its owners lost in time. However in the 1990s the then director of Spain's Archaeological Museum, José M Luzón Nogué, began to research some puzzling marble candelabra in the collection, purportedly antique, but to the tutored eye palpably later fabrications from ancient fragments. Where had they come from? When were they made? And for whom?
His research, according to Whistler, led him back through a "trail of 18th-century documents" kept by a punctilious Spanish bureaucracy, and revealed their origin as part of what was called the Presa Ynglesa – the English Prize, the loot from the Westmorland. And finally, using inventories and receipts to identify a panoply of up till then mysterious objects scattered through Spain's museums, scholars have completed what Whistler calls this "vast jigsaw puzzle".
From next May, the best of the "English Prize" will be assembled and shown at the Ashmolean – bringing visitors a unique time capsule of the tastes, characters and groaning shopping bags of Britain's 18th-century super-rich gentlemen. For scholars of the Grand Tour, it shows not only what art the grand tourists were buying, but also what books they were reading to while away the journeys and what souvenirs they were sending back for their friends and families. It provides the most complete picture yet of an influential moment in the history of British taste and learning.
The most extravagant of the travellers was Francis Basset, the heir to a Cornish tin-mining fortune. He had travelled down to Rome and Naples through France, and having sent possessions back to England on board the Westmorland, he himself then travelled overland to Venice to continue his tour. Evidence of his outward route comes from a guidebook – A Relation of a Journey to the Glaciers in the Dutchy [sic] of Savoy – which would have helped him and his tutor through the Alps.
In Rome, Basset commissioned a portrait of himself by the artist Pompeo Batoni, the artist to commission at the time. That work – which portrays Basset leaning on an antique sculpture, with St Peter's in the background – had hung, its sitter unidentified, in the Prado in Madrid until the recent research.
Basset also bought 14 volumes of engravings by Giovanni Battista Piranesi and a series of works by John Robert Cozens. Chief curator at the Yale Centre for British Art in Connecticut, Scott Wilcox, where the exhibition travels next October, described these as "the missing link in the development of one of the greatest watercolourists of the 18th century".
He also sent back a batch of scores of Haydn string quartets, and his copy of the latest experimental novel – Laurence Sterne's Tristram Shandy – testament to the breadth of his chic and learned tastes.
The Westmorland, says Whistler, is "full of stories". One traveller, a Scot named John Henderson, sent back a portrait of a beautiful, rather sulky, looking young man, sitter unknown. He had made strenuous attempts to recover it, an intermediary later writing to the British embassy said that "there is a Portrait he joys [sic] which he wishes much to have". Even hard-nosed scholars have been led to speculate that there might have been a love affair in the case.
On board was also a collection of saints' relics, concealed (bound as they were for Protestant England) within a marble plinth. They had been hidden there by an English Jesuit based in Rome, John Thorpe, who was sending them to the 8th Lord Arundell of Wardour, a prominent English Catholic. After the Westmorland's capture, a Papal nuncio "had to scramble to get them back before they were unknowingly sold", said Wilcox.
But will any of the Basset and Henderson descendants be coming forward to reclaim their ancestors' belongings from the Spanish state when this captured loot finally returns home? No, according to Whistler, the booty was, apparently, declared a legal prize of war.
The English Prize: The Capture of the Westmorland, an Episode of the Grand Tour, is at the Ashmolean, Oxford, from 17 May-27 August next year.
• This article was amended on 29 November 2011. The original referred to relics being sent to the Earl of Arundel. This has been corrected.