I met the real Lady of Downton Abbey and she revealed huge catastrophe on first day

Aristocrats and A-listers visiting the lavish real-life set of Downton Abbey usually arrive in a chauffeur driven car with an assistant on hand for any luggage. On the day I meet the 8th Countess of Carnarvon at Highclere Castle, I'm clad in an elegant designer top with my hair perfectly coiffed - but, thanks to my supermarket bag-for-life and mini-cab, there's no disguising my commoner roots.

Highclere is, of course, the country estate in Hampshire recognised the world over as the majestic main character in Julian Fellowes' epic drama. There might not be a line-up of staff on the drive to greet my less than aristocratic entrance, but nonetheless I'm nervous.

I'm about to come face to face with castle owner Lady Fiona Carnarvon, a real life blue-blood who, just a 100 years before, would only have glanced my way to check I hadn't missed a spot of dust on the silverware.

It's thanks to social mobility that I am here at all. And as we know with nobility, first impressions count. Lady Carnarvon can be found perched on the corner of a desk inside an ordinary office room where her closest household allies help her plan the day ahead.

She's busy getting to grips with the nitty gritty of her hectic schedule amid a cacophony of ringing mobiles. Like a below-stairs maid who has taken a wrong turn, I stand nervously resisting the urge to curtsy. I extend my hand for a firm (but not too firm - as a school careers advisor once counselled) handshake and introduce myself as "Sam", a name the upper classes admittedly struggle with at the best of times.

Lady C, as she likes to be known, politely asks if I have travelled far. "Essex", I respond, waiting to see if smelling salts might be required. "Just like Michelle Dockery."

The smile she shoots back tells me I never had any reason to worry.

For the last 15 years, Downton Abbey has captivated millions of fans with its dramatic storylines, stunning sets and witty dialogue.

Featuring screen stalwarts Dame Maggie Smith, Hugh Bonneville and Jim Carter, it also launched the careers of the aforementioned Michelle Dockery, Lily James and Dan Stevens. Now, after six seasons and two successful films, it's finally ending with its third and final cinematic instalment, The Grand Finale, released on Friday [Sept 12].

To coincide with the momentous finale, Lady C has written A Year At Highclere - Secrets and stories from the real Downton Abbey, providing a candid account of life, now and then, inside the famous centuries old estate with a vivid story of its own.

And the 61-year-old, who moved to the grand estate in 2001 with her husband Geordie, the 8th Earl of Carnarvon, following his father's death, has granted the Daily Express an exclusive interview to share her filming secrets.

Highclere's records stretch back to an astonishing 748 AD with a lineage that includes the renowned Egyptologist Lord Carnarvon, who discovered the tomb of Tutankhamun - and, as legend has it, succumbed to its infamous curse.

The responsibility of stewarding a Grade 1 listed national treasure is immense and Lady Carnarvon and Geordie are acutely aware of this. "I hope we come across as just a hard working couple, you know, ones who graft like everyone else," she says.

And graft they do. When film crews descend upon Highclere, it's a meticulously orchestrated affair. "They arrive at 6am outside and 7am inside the house," she explains. "So you hear all the vans drive up."

While her husband, Geordie, handles the initial welcome, she uses the early hours for emails before the 11-hour filming day truly begins.

Constant coordination with the housekeeping team and film crew is required to ensure rooms are ready and valuable artefacts are kept safe at all times, as Lady C explains.

"One of the state rooms is always off limits as it has a signed fire exit in it, so that provides a really good excuse to keep it to ourselves - and that's the side of the house that has everything that shouldn't really be touched by anybody such as vases and other heirlooms."

Inevitably, there are mishaps.

On the evening before the first day of filming in March 2010, she and Geordie made sure they moved their most prized possessions to a safe place.

But they forgot one item. "We noticed a green-lacquered objet de vertu (an antique box) on a table opposite the fireplace," explains Lady C.

"I assured Geordie it would be fine but it didn't last the day as a lighting boom fell on it - the crew were mortified and it took six months for me to find someone able to repair it. I felt dreadfully guilty but my husband very kindly did not say, 'I told you so'."

She adds with a knowing smile: "We always say you must have a good sense of British humour to work here. If you get on with Fawlty Towers and The Two Ronnies, then you sort of fit in at Highclere, basically."

Much of their glassware and crockery was rented after that. But, over the next 15 years, the cast became friends, with many returning between filming to stay at Highclere as guests of the family. Lady C soon learned that an actor's life consists of a moment's deliverance following an age-old wait.

"We often saw Jim Carter hanging around outside while the others were filming," she laughs. "He is a really nice man. He'd have to sit outside all bloody day waiting to say, 'You rang M'Lord', or whatever."

But she soon learned it's not about which actor has the most lines or most camera time, but what they do with it.

"Maggie Smith always made her words fill out," she remembers. "I learned so much from watching her. She didn't have a huge presence in any of the episodes because she was filming other projects including Harry Potter and The Best Exotic Marigold Hotel, but yet with just one line, or the one scene that she did, everybody remembers it - it was like magic."

Prior to Downton Abbey, Highclere Castle was used in Stephen Fry and Hugh Laurie's 90s comedy television series Jeeves and Wooster and in scenes for the 1987 film The Secret Garden. Now Downton is ending, would Lady C consider opening her home's doors to another filming crew?

"Highclere matters to me," she replies. "We have had various calls recently, and they're just not what I think is right, especially not for those who love Downton Abbey.

"I meet people here who have watched it 22 times and it means something to them. I receive emails from fans of the show with stories about how their loved-one is not at all well and visiting Highclere is on their bucket list before they die. I don't want to muddy that with some reality TV show or a project that doesn't respect the Downton legacy, why should I? It means too much to people for me to allow it to unravel."

I personally witness the hordes of paying visitors as Highclere is open to the public five days out of seven. To escape the crowds, Lady C takes me out to what is commonly regarded as Lady Mary's bench in Downton.

But before long, a small crowd begins to form, having recognised the true lady of the manor from previous interviews. She leaps from the bench to spark a conversation with those gathered, instantly putting them at ease.

After several exchanges and photos with visitors from around the globe she explains her goal is to "make everyone feel welcome".

Lady C continues: "I simply love that people can cross the threshold here and wander through the gardens. If you wanted privacy, you would definitely be in the wrong job.

"Being at Highclere it is so important to get on with everyone and it's been that way for centuries. Visitors and staff - it's a big community here and that's the joy of it."

The castle has received many VIPs in its time, including George Clooney who filmed a special ITV charity edition of Downton in 2014. As Lady C writes: "I sincerely believe that most women who worked at ITV came to Highclere that day. So did the press, but only as far as our gates, as they thought Mr Clooney might be considering Highclere as a venue for his wedding to then fiancee Amal."

But reflecting on Highclere's open-door policy now, she says: "We are all part of the same plan - we need people to come and enjoy it so the estate can survive. I'm sort of working for them (the staff) as well, to make sure that every month we have enough money to pay them so that they can pay their mortgages. We all need each other."

In fact, there's a big community relying on the profitability of the house.

"There's the banqueting team - 1,200 people that serve food, there's the team in the kitchens who are cooking the food for the banqueting teams, there's the office, which is far too small, gardeners and the list goes on," says Lady C.

Despite their best efforts, she acknowledges that Highclere does "sometimes get it wrong". Its inbox is a source of much needed feedback from visitors and occasionally there is criticism. With her head in her hands, she confesses: "Paper cups! People do complain about the fact we don't serve our hot drinks in the fine bone china they expect from a set up like ours but it's just not viable!

"After Covid we moved to a recyclable option and we have stuck with it because of the cost and manpower required to keep a clean supply of china - but it doesn't go down well with some. As a compromise we have started stocking paper cups with a lovely map on Highclere on it so hopefully that will soften the blow for anyone expecting a full tea service - and we do offer afternoon tea as that is a must-have."

The plot of the final film has been firmly kept under wraps until its release. But the action, now set in the 1930s as the Crawleys tend with financial turmoil and the fallout from Lady Mary's scandalous divorce from Henry Talbot (Matthew Goodwin) will keep the drama stakes high.

As for her own future, Lady C is philosophical. "There's no point doing something like Downton if you don't make the best of it because otherwise there has been nothing in it for us," she says. "So, in the end, you do need something called cash."

And with a team the size of Highclere's to provide for a very busy Lady C may well find herself pondering Maggie Smith's famous question in the series, "What is a weekend?"

• A Year At Highclere: Secrets and Stories from the Real Downton Abbey, by the Countess of Carnarvon (Century, £22) is out now