THORNTON HALL is a place of blocked up windows and secret tunnels. It has a lost history, an avenue of trees that leads nowhere, and a rarely seen ghost.

But it is also a place of open gardens, of beautiful views, of colourful displays, and of afternoon teas – and it is a place that is open on Bank Holiday Monday, raising money for charity, with a plant fair in its courtyard.

Thornton Hall is on the outskirts of Cockerton. It is one of just seven Grade I listed buildings in the borough of Darlington, which means it is in the top one per cent of most important buildings in the country.

There has probably been a manor house beside the Staindrop Road since time immemorial, although our story really begins around 1550 when Robert Tailbois, of Hurworth, married the heiress of “Thornton of Thornton” and so acquired the Thornton estate.

His son, Ralph, inherited Thornton. In 1569, during the Rising of the North, many leading members of the Durham gentry supported the Catholic rebels, but Ralph remained loyal to the Protestant Queen Elizabeth – he was a captain in her armed forces, earning eight shillings a day. For his loyalty, Ralph was rewarded with the neighbouring estates of Ulnaby and Carlbury, and with his new-found wealth, Ralph built the southern wing of Thornton Hall – the face that greets visitors to the gardens.

Indeed, it may even be Ralph’s fierce gargoyles – “right ugly nondescript animals”, according to a Vcitorian historian – which stare down on visitors as they approach the front door.

The initials of Ralph and his second wife, Jane Bertram, are to be found carved elegantly into ancient beams at Thornton, and his coat of arms has been moulded in plaster on the ceiling of the Long Gallery – the same coat of arms can be found on the tower of Hurworth church.

Ralph died in 1591, and his son, another Robert, took over Thornton. He married a daughter of the Bishop of Durham, but not even that could save him – he died in 1606 a prisoner in Durham jail, guilty of an unrecorded misdeamour (probably disloyalty to the monarch), but left no heir, and so for about a decade Thornton Hall was owned by the Salvin family of Croxdale.

In 1620, Henry Bowes, a merchant adventurer who was Sheriff of Newcastle, bought Thornton Hall, and it was his son, Sir Francis, who added the northern half in the 1630s.

This substantial extension, including a grand oak staircase, is said to have been necessary to satisfy Sir Francis’ third wife, Margaret Delaval, whose family hailed from the palatial surroundings of Seaton Delaval Hall, in Northumberland.

One of the most striking features of Thornton Hall is how many of its grand, mullioned windows have been filled in to avoid being liable to the window tax, which was introduced in 1696.

The last of the Bowes family to live at Thornton was George, who died in 1752, leaving three daughters. One married Lieutenant-Colonel Thomas Thoroton of the Coldstream Guards; another married the Reverend Robert Croft, of Stillington, in North Yorkshire, while the third, Margarett, didn’t marry at all. Their inheritances were rolled into the Thornton and Croft Trust which still owns the hall, and which, over the centuries, has rented the hall to farmers.

Since 1752, therefore, Thornton hasn’t been the home of one of the leading industrialists that so made their mark on a borough like Darlington. As a consequence, it has punched beneath its weight, and its tenants have at times allowed it to flirt with dereliction.

When the Manners family first let it in 1947, the formal gardens of the Tailbois family had disappeared as the cows and sheep grazed right up to the back door.

“At first, I just wanted somewhere where the children could play without getting covered in cow muck,” says Sue Manners, who joined her husband, Michael, in the hall 30 years ago. “We got a swing and a slide, and then we made a tennis court, but the bounce was terrible.

“I got my first small greenhouse in 1994, but I didn’t know a thing about gardening.”

The turf of the cows’ fields was dug up, and first of all the Tailbois’ 16th Century garden with its raised beds leading up to a belvedere was rediscovered, and then a large less formal garden, with a wildlife pond, was developed.

“It’s a hobby that’s got out of hand,” says Sue. “I just got hooked – you’ve got to have something in life, and it’s just as well I didn’t like posh clothes.

“I just had a blank canvas, with no masterplan, but there’s nothing right and nothing wrong.”

The garden was first opened, to acclaim, in 2005, and now opens a handful of times a year before harvesting becomes the focus of the farm’s efforts. Homemade lunches and afternoon teas are available, and this year there is a fine art bronze sculpture exhibition.

Some visitors come to admire the plants that are Sue’s handiwork; others come to soak up the history of this marvellous hall.

And everyone wonders at the avenue of limes in the field opposite that leads nowhere, at the arch in an outhouse which is said to be the start of a secret tunnel to Walworth Castle more than a mile away (there is another arch in a cellar at the castle where the tunnel is said to come out), and at whether a strange rustling sound is the silken dress of the White Lady who haunts the upper floors or just the wind in the leaves of the “veteran mulberry” tree that has for centuries stood at the entrance to this historic, but rather hidden, place.

THORNTON HALL, on Staindrop Road (DL2 2NB) is open on Monday from 10.30am to 4pm. Admission is £7 per person, and all proceeds go to charity: this year the Great North Air Ambulance, Alzheimer's Research, and Prostrate Cancer. It is also open on June 13, 20, 27 and July 4, 11 and 15.