Meet Lady Carnarvon of Highclere Castle - the real head of Downton Abbey and Mary Berry’s new friend

"She was enormous fun," says Lady Carnarvon, who got along jolly well with Mary. "She's just so admirable because she is so hard working. I think she is a complete beacon."

And everyone's favourite baking matriarch reminds her of another determined older lady: "I've watched Maggie Smith up at silly o'clock to get into costume and make-up for Highclere – for Downton Abbey – sorry, the whole thing gets so muddled sometimes."

It's not surprising she's in a bit of a muddle, because as she says, "we are Downton Abbey". Not that she minds at all: for Highclere, Downton was the opportunity of a lifetime.

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This castle may have stood for centuries, with people living here for over 1,300 years, but television has transformed its place in the world. On screen, the Earl of Downton and his family spent multiple series wondering about the place of a stately home in post-First World War Britain, making hard decisions about diversifying their estate and cutting down on servants – but in the 21st century the Carnarvons have hit upon a solution.

Having already hosted filming for Jeeves and Worcester, The Secret Garden and Inspector Morse, Lady Carnarvon wasn't expecting Julian Fellowes' Downton Abbey to change her life. But it became an instant success and ITV's period drama beamed images of her drawing room and dining room and library into millions of households around the world.

How is it to see your home on screen? "It always makes you feel slightly odd," Lady C admits. "But sometimes when I go and give talks somewhere away, they show bits of Highclere on TV and I'm standing 3,000 miles away in America, and suddenly I've got a huge group of people in front of of me and they think they know my home."

Filming took place in the "upstairs" part of the castle, but a set was created to use as the servants' quarters. It was seamless. An enthusiastic Lady C explains: "I love the magic of film. When Jim Carter – Mr Carson – started walking down our staff stone staircase here, they rebuilt the bottom five or six steps on a film set, so two weeks later he'd come out in Ealing Studios! I love that. And I find the whole process of creating stories and content for people to watch and enjoy really fascinating."

The Downton years really were an extraordinary time in Highclere’s long history, and also in the lives of Lord and Lady Carnarvon and the three children they have between them: Lady Saoirse Herbert, Lord Porchester and the Hon Edward Herbert.

Every February the cast and crew would rock up and make their home at Highclere, which sits in Hampshire near the town of Newbury. "There was that sense of the school year," Lady Carnarvon says. "It was quite relaxed to start with, and then as it got towards the final days, they then have to stop, edit, and present it on TV. Just like working for your A-Levels or exams! A blooming nightmare because there was no opportunity to run over schedule."

Summer would come and the pressure would rise until the first week in July, when filming would finish with the Christmas special. And the sun would come out at exactly the wrong moment. "That would be the one week when it was really hot and sunny and they'd be dressed up singing Silent Night!" Lady Carnarvon says.

Life at Highclere during filming was often rather surreal. Take the projectile-vomiting-exploding-ulcer incident, when Hugh Bonneville's Earl of Downton abruptly stood up at dinner and splattered his horrified dinner guests with blood. "I'm not sure that Elizabeth McGovern realised how much of the blood was going to come out," Lady C reveals. "And they could only do it once because of all the set-up for it.

"We had blankets underneath the table and it's their white tablecloth on top, you'll be happy to know. And then on our carpet underneath we had again blankets and protection.

"But I would always hope that any client – and Downton were a very special client – would get the best possible shot they're trying to achieve, what they need for their purposes, and I'm trying to make sure it's all still standing for the next day whether it's for them or whether it's for someone getting married here."

That's Lady Carnarvon switching from Downton Fan Mode into Business Mode. Highclere does weddings, tours, and Christmas fairs; it welcomes the public for high teas and special dinners and concerts. On the estate you have a team of people training gun dogs and organising shoots for paying guests, and plenty of others busying themselves around the castle keeping it spic and span. Highclere is a business like never before.

And then there's the Ancient Egyptian connection – another impressive entry on Highclere's bulging CV.

If the name Carnarvon sounds familiar, you probably heard it in primary school (or on another recent Sunday-night ITV period drama). That’s because the present Lord Carnarvon is the great-grandson of that Lord Carnarvon who funded the discovery of the tomb of Tutankhamen 95 years ago.

The fifth Earl of Carnarvon keeled over and died of blood poisoning shortly after the tomb was unearther (or was possibly struck down by the Mummy's Curse, depending on who you believe). But that's not the end of the story, because when the present Lord Carnarvon (the eighth Earl) inherited Highclere, he discovered it was absolutely packed with hidden treasure.

“It was the front page news of The Times, because the butler Robert Taylor and Lord Carnarvon – my husband – found the remains of the fifth earl's Egyptian collection tucked away behind drawers, and in cupboards and cellars,” Lady C says, adding with the art of understatement: “It was very fortunate.” Nowadays, visitors flock to see the exhibition of Egyptian treasures within the castle walls.

The Carnarvons have become expert at making the most of their house's history. They've even had a champagne developed in France according to the tastes of their famous visitor Bertie, the Prince of Wales, who popped in for a lavish weekend of shooting in 1895.

As for herself, Lady Carnarvon has jumped on the Downton bandwagon with a couple of books: At Home at Highclere, and Lady Almina and the Real Downton Abbey. On tours, you can inspect the desk where Bonneville wrote his letters – and the tour guides will happily provide all the anecdotes they can, having hung around set helping to move chairs and listening in.

“I’m following my nose,” Lady Carnarvon says, and she must have a nose like one of the gun dogs from the Highclere Estate because business is flourishing. She adds: "It has been good, but it's what you make of it. Highclere has had a career break and we're working hard to see how we can make best use of it."

Also following her nose all the way to Highclere is Mary Berry as she kicks off her new BBC series, Mary Berry's Country House Secrets. It brings together the twin British kitschy delights of the Great British Bake Off and Downton Abbey – and in Lady C she finds a kindred spirit who has a delight in food.

Lady Carnarvon and Mary Berry get on famously. They both find everything "glorious" and "magnificent" and Lady C settles straight in to being Berry's sous-chef. "She did write down and give me one of her tomato tartlet recipes," Lady C says, before embarking upon a monologue about how wonderful tomatoes are (she really loves tomatoes).

They are both no-nonsense countryside types, with a love of simple, wholesome food, as well as posh food made with care and dedication. At the end of the episode Mary gets to attend one of Highclere's famed dinner parties, a tradition stretching back 200 years, and before descending those famous stairs she proclaims herself to be Lady Mary (well you wouldn’t expect her to be the Dowager Countess, would you?).

Will they keep in touch? "I'm sure we will," Lady Carnarvon says. "Mary Berry was enormous fun as a party guest. It's lovely to sit down and relax with her, although it's not always that relaxing with the cameras around. I hope she'll come back and have lunch with me another time out of the glare of everything."

But next time Mary comes back, she may well bump into the other Lady Mary in the corridor. There have been whispers of a Downton movie for years – in fact, the whispers have reached the level of "deafening chatter" by now.

Have the Carnarvons been approached about a movie? "I'm not allowed to say."

Well then – would she be up for it, if they did? "I am sure that anyone who has been fortunate enough to be associated with Downton, with the warmth, the fun, the sense of enjoyment it created for viewers – obviously we would all try to make it work. And Downton – or Highclere – is at the heart of it," she says.

And the timing is apparently right: "I would love it... it's a little bit of escapism in what is quite a tricky world at the moment, and it's a lovely thing to watch. It gives you a sense that there is hope."

Still – if you're missing Downton before Julian Fellowes gets his act together for a feature film, at least you can take a trip to Highclere with Mary Berry. And Lady Carnarvon is pleased to welcome more curious viewers to her home in person, as Highclere's "career break" continues.

Mary Berry's Country House Secrets begins on Wednesday 22nd November at 8pm on BBC1