A remarkable study for Flaming June, one of the best known of all Pre-Raphaelite paintings, has been discovered hanging discreetly behind a bedroom door in an English country mansion.

The discovery of the head study for Sir Frederic Leighton’s picture was announced on Friday – one of many extraordinary secrets to emerge from a 16th-century manor house owned by Mary, Duchess of Roxburghe until her death, aged 99, last year.

The heir, to his immense surprise, was her great-nephew Bamber Gascoigne, former host of University Challenge.

He has vowed to begin carrying out the essential restoration needed to secure the house’s future and has arranged with Sotheby’s to sell objects from the house which paint a picture of an England that no longer exists.

The Leighton drawing is particularly exciting. Simon Toll, Sotheby’s Victorian art specialist, said finding it behind the door of a small, dark anteroom off the duchess’s bedroom was “thrilling ... one of the most heart-stopping moments in my career”.

Sir Frederic Leighton’s 1895 painting Flaming June. Photograph: Museo de Arte de Ponce

Flaming June is on posters all over the world, yet resides in the most unexpected of places – the municipal art museum of Puerto Rico’s second biggest city, Ponce, where it is known as “the Mona Lisa of the southern hemisphere”.

It was snapped up by the governor of Puerto Rico, Luis Ferré, in 1963 for $1,000 (£660) when Pre-Raphaelites were still painfully out-of-fashion. But while experts were aware of the existence of the pencil and chalk preparatory drawing because it featured in an 1895 art magazine, they did not know its location.

Toll said he immediately recognised the study, which joins existing nude and drapery studies for the painting. “This head study is the last piece of the jigsaw in terms of the preparatory work Leighton undertook before starting on the big oil painting.”

It seems likely that the drawing was purchased directly from the artist’s studio after Leighton’s death.It is just one treasure from West Horlsey Place, the duchess’s remarkable time-capsule house – to walk around it is like being in an EM Forster novel. .

She allowed few people beyond the main reception rooms. Gascoigne, now 80, spent many happy lunches at his great-aunt’s house but never stepped foot past the stone hall where meals were served or the drawing room and garden room.

“In all that time she never said to anybody, ‘Would you like to see upstairs?’ I think it may have been considered bad form, showing off or something. I saw the amazing upstairs drawing room for the first time, as its owner.”

He had no idea that such a treasure trove existed in the labyrinth of rooms beyond.

The contents evoke a lifestyle so aristocratically excessive that characters in Downton Abbey might have thought it a touch too much. There are liveried staff uniforms; monogrammed china which members of the royal family would have used as dinner guests; a 10ft white cut-velvet cloak studded with paste stones which the duchess’s mother would have worn while welcoming guests to parties in the 1920s and 1930s; and a silver Asprey breakfast-in-bed tray.

The duchess’s life was as fascinating as her house. She was born Mary Evelyn Hungerford Crewe-Milnes and married the Duke of Roxburghe – “Bobo” to his friends and family – in Westminster Abbey in 1935, a society wedding that brought together two of Britain’s great aristocratic families.

The marriage deteriorated, and in 1953 her husband had his butler deliver divorce papers to her on a silver tray while they were both eating breakfast.

Not best pleased, Mary barricaded herself into a castle wing where she remained for two months, in spite of the duke turning off the power and water. It was only the intervention of a neighbour, the Earl of Home, soon to be prime minister Alec Douglas-Home, which brought the standoff to a close.

Mary then divided her time between London and West Horsley Place, purchased by her parents as a country retreat in the 1930s.

Gasgoigne was not expecting to inherit it. “I was absolutely astonished and, in a way, it didn’t mean anything for a bit – it seemed so strange. It is beginning to make sense now after a year or so.”

The house, essentially Tudor era with an 18th-century red-brick facade screwed to it, is beautiful but needs extensive restoration.

“People who know about houses say ‘I do sympathise with you,’” said Gascoigne. “They say ‘thank God it isn’t me’.”

The contents sale on 27 May is estimated at £1.7m-2.5m, with jewels being sold at a separate sale in Geneva. Flaming June will be sold at sale on 15 July.

Whatever money is made will, after death duties, be ploughed into the house. “It is rather a late age in life to be starting an adventure,” admitted Gascoigne. “Having failed to climb Everest perhaps one needs to try something else.”