A LOST Teesdale castle is the focus of an exhibition at the Bowes Museum which opens on Saturday. Once Streatlam Castle was the home of the tenth richest man in England. He filled it with some of his most prized and personal possessions, and he used its parkland to breed some of the finest racehorses of the Victorian era.

The castle, though, was blown up by the Army in 1959, and today dotted around the parkland are lots of little bits that hint at once was the home of the museum founders: there’s an orangery, an archway, a monument to a racehorse, a collection of exotic trees, a culverted stream, a leg-mangling mantrap…

All of this is pieced back together in the new exhibition, which has a pair of intricate models of the lost castle as its centrepiece, showing the extraordinary scale of the home of John and Josephine Bowes.

And there’s a mantrap, which might once have caught poachers sneaking into their grounds.

Streatlam Castle, to the north-east of Barnard Castle, was the home of the Bowes family from the early 14th Century, and its ups and downs mirrored their ups and downs. When John Bowes-Lyon, the 10th Earl of Strathmore inherited it in 1776, it was “fast falling into decay and ruin”, but he started breeding racehorses there. His other love was Mary Milner, a humble gardener’s daughter from nearby Stainton – one of the exhibition highlights is a portrait of Mary from the private apartments in Glamis Castle.

In 1811, she gave birth to a son, John, and in an attempt to make the boy legitimate, she married her earl on his deathbed in 1820.

The earl died 16 hours later, the nine-year-old John inherited Streatlam but, after a bitter court battle, lost his Scottish title due to his illegitimacy.

“As soon as he inherited the castle, there was a blizzard of plans to improve it,” says Jonathan Peacock, who has curated the exhibition. “It was a rather plain and unadorned building and he stuck the portico and cupolas on it and turned it from a castle into a mansion.”

John’s coal-rich lands at Gibside in north Durham brought him an annual income of £3m in today’s values. Then came proceeds from horseracing: in 1843, Cotherstone, bred at Streatlam, won the Derby – not only did John collect the £4,250 prize but he won bets worth at least £21,000. He earned as much from the horses that afternoon as he did from his land in a year, although he was forced to lie low in France for a year to avoid being prosecuted over an allegedly underhand gambling tactic.

Paris exerted an increasing hold on him during his thirties, as it allowed him to escape the conventionalities of Durham and the stigma of his illegitimacy. He bought a theatre and then a chateau and in 1852, he married an actress, Josephine Benoite Coffin-Chevalier, 13 years his junior. They began buying up artistic artefacts, storing them in the chateau, with the madcap idea of establishing a museum.

“John clearly loved Streatlam Castle – he had been brought up there and it was always home to him, but he spent very little time living there,” says Jonathan. “He was a very absentee landlord, and when he was living in Paris, he was writing two or three letters a day to his agent, Ralph Dent, with copious instructions about the castle.”

Many of those letters were about the Streatlam parkland, and the items in the castle, which were his personal collection of art and furniture, which he had either inherited or bought pre-Josephine. His collection with Josephine was very different, and always intended for the museum.

In 1860, they sold the chateau on the banks of the Seine and began to decamp to Streatlam, to establish the museum. Their orangery came with them – a dozen or so orange trees transported in specially-built packing cases by barge to Middlesbrough and then by road to Streatlam, a crew of men lopping off branches of overhanging trees along the A67 to ensure their safe arrival. The heated building that John created for them is one of the few pieces that survives at Streatlam.

Sadly neither Josephine nor John lived long enough to see their museum, in a giant French chateau, open in 1892, although the exhibition about their British castle is one of a series of events celebrating the museum’s 125th anniversary.

Following their deaths, the Bowes-Lyon family found Streatlam castle surplus to requirements. It was sold in 1922, and effectively striped and abandoned until on March 29, 1959, the Territorial Army was allowed to do an exercise involving dynamite on the site, and the Bowes family home for six centuries disappeared in a rumble of explosives and a puff of smoke.

“The exhibition has allowed us to re-evaluate what remains, and in researching it, we’ve learned so much more about John and Josephine Bowes and the extraordinary collection they have left us in the museum,” says Jonathan.

BLOB The Streatlam Castle exhibition runs until March 11, 2018, after which it transfers to Glamis Castle