The walls of Althorp House are lined with portraits of its inhabitants past and present. This is a house with a designated Picture Gallery, but there are 92 other rooms too, almost every one rich with oil paintings.

At the top of the grand staircase in the middle of the house are imposing portraits of Charles, the current Earl Spencer, his father and a famous one of the late Diana, Princess of Wales, who grew up in this house and used to practise her tap-dancing on the black-and-white tiles of the hallway.

The sense that you are touching history is powerful, but there is also a conundrum here.

Do you have to have a portrait done now, I ask the newish chatelaine of Althorp, Professor Cat Jarman. How does this work?

Earl Spencer’s partner of two years – a woman who arrived here with a pick-axe on an archaeological dig – shudders a little, as she gazes upwards.

Norwegian academic Cat, 44, isn’t even comfortable having her photo done (‘are jeans OK? Do these boots match?’) so the idea of her posing for a formal oil painting, maybe sweeping down this staircase in a ballgown or a big hat, seems just too bizarre.

‘Not really my thing,’ she says. ‘And I’ve thought about this. This is Charles’s world, and it’s obviously very interesting, and I’m respectful of it, but I don’t feel that this place is about me. I don’t feel any need to have a portrait up, or make a mark on it. I find it quite... quirky but odd.’

Dare I ask if her predecessor, Earl Spencer’s third wife, Karen – a woman who did like a hat, and with whom there have been some issues, legal and otherwise – ever had a portrait done? A slight pause. ‘There was one, but it’s no longer here,’ she says.

Karen, it transpires, also had her initials woven into the carpet at the top of that grand staircase, which had to be rewoven as the monogram was intended to be permanent.

We are meeting in the week when a line was drawn under the Karen issue, and – in theory – everyone can move on. Last Thursday, Cat Jarman walked out of the High Court in London feeling vindicated. For the past year, all parties here have been embroiled in a legal ding-dong that has run alongside already acrimonious Spencer divorce proceedings.

It came about when Cat took the extraordinary step of suing Karen (still Countess Spencer) for disclosing private medical information about her – namely that Cat had multiple sclerosis, something that only a few people in her life knew about.

At that stage, in 2024, Charles Spencer – her boyfriend of only a few months – was not one of them.

Cat had been diagnosed in 2016, while she was doing her PhD. It had been devastating for her, caused her to change career, and – crucially – she had chosen (‘as is everyone’s right,’ she says today) to keep that information to herself, lest it colour how colleagues treated her.

She was completely floored, she admits, to discover that Karen not only saw fit to tell Earl Spencer, but staff at Althorp, and even staff at her daughter’s school.

The legal tussle has been going on for a year but was settled last week before it went to trial, and a statement in the High Court concluded matters. Not before Cat had effectively been publicly branded a homewrecker, though, a woman who waltzed into Althorp and, metaphorically anyway, took a pick-axe to a marriage. Actually, two marriages. That simply wasn’t true, she says today.

‘That was so hurtful, and anyone who knows me knows it wasn’t true. Both marriages were over when Charles and I began a relationship, and had been for some time. Tom and I had been separated for years, but we’d just kept it quiet for the sake of our children. We’d co-existed perfectly amicably, but the marriage was over.’ Yet Tom – who last week publicly backed his ex-wife, saying he had been misled by the countess – became embroiled when he was told, by Karen, that she had unequivocal proof that Cat was having an affair with her husband.

Again, not true, insists Cat. ‘Tom was told there was evidence, proof, which there wasn’t, and all this was weaponised.’

He was so ‘distressed’ because in speaking to Karen he had divulged his former partner’s health condition which she then used against the couple. ‘The whole thing had a devastating effect on our whole family,’ she says. ‘It’s taken a long time to put together the pieces.’

She’s particularly furious that the legal system was used to brand her a husband-stealer. ‘These accusations are very difficult to disprove anyway, but it was the way it was done – putting it in legal documents, where it could be reported without risk of being accused of libel, which she would have been had she said that in, say, a newspaper.’

It’s still a complicated issue, because Earl Spencer was ordered to cover some of his ex-wife’s costs, but Cat admits today that ‘I feel it is very much a victory, and the statement in court was a chance to tell my side of the story in a formal way. I got to set the record straight and speak about the impact it had on me. It’s been a challenging year, to say the least.’ For her part, Karen does not accept any wrongdoing. Why did Cat take the action in the first place? ‘It was about fairness. It wasn’t right that she shared private medical information. Sharing something [like an MS diagnosis] should be a matter of choice.’

Do you understand Karen’s anger and hurt, and desire to lash out, I ask? ‘Not necessarily, no. I don’t think I do really. The [Spencer] marriage was already over. It was nothing to do with me. But I guess she wanted a narrative that suited her means.’

And you were the required villain? ‘Yes, I think I happened to be in the wrong place at the wrong time, and it is very hurtful when there isn’t the slightest element of truth in it. It just feels really unnecessary, and the fact it was so public. It’s just upsetting.’

What’s the relationship with Karen like now? ‘I don’t have any contact with her. I hope that this can be the end of it now. We can all live our separate lives.’ Astonishingly, Cat – who already has a whole clutch of academic degrees – is actually going back to university in the wake of this.

‘I’m about to start studying for a law conversion degree because of this. I’m just interested in how the process works.’

This is Cat’s first interview at Althorp. Initially, she comes across as the most unlikely lady-of-the-manor ever. As we meander past marble statues and busts on plinths to Althorp’s epic library (think the library of Belle’s dreams in Beauty And The Beast), she admits that it still doesn’t feel quite like home.

‘I come from a country where we don’t have any of this. We don’t have an aristocracy, really. Even the royal family in Norway – no one cares about them very much, certainly not in the same way.

‘It’s very strange to be living here, fascinating but strange. I’m a normal person plonked in an abnormal world. If Charles said to me, “I’m going to give all this to the National Trust”, I would be entirely content. I would live with him in a hut.’

Her association with Charles – and this house – began as a purely professional one. Filming at Althorp, she got chatting to the earl, exchanging ‘nerdy’ history talk (he is also a historian and author). They ended up hosting a podcast together, with their friend, the author and broadcaster Rev Richard Coles. Never had he laughed so much, apparently. And she found herself laughing, too.

It was absolutely not love at first sight, she says, but does reflect on how their friendship developed at a pivotal time in Charles’s life.

He had been writing his memoirs at the time – a painful and ‘life-changing’ (her words) process, which led to a public revelation about being abused at school, and a kind of breakdown. She was there to offer support and advice in a way, perhaps, that his wife was not.

Although the countess has since suggested that the dates were suspect, both Charles and Cat insist they were not. The Spencer marriage was over long before romance developed between Charles and Cat.

‘We were very much friends for a long time,’ she says today. ‘Neither of us was looking for anything else. We really weren’t. I think that was the key. And the timing was significant for both of us. We both needed something new, but maybe didn’t realise we did.’

Still, you don’t need a PhD to know that stepping into a relationship with Charles was likely to come with complications. After all, she was a Scandinavian academic who wrote books about Vikings, so falling in love with the thrice-married earl who has seven children was, if nothing else, the talk of the aristocratic set.

‘It’s a complicated life to come into,’ she agrees. ‘It wasn’t something I ever thought I wanted, but I think it’s significant that he was going through this life-changing process when he was writing his book. I think it was the catalyst to him seeing who he really was and who he wanted to be.

‘Also, why he’d had these different relationships in the past. Ultimately, when you fall in love, you see the person in front of you. They are who they are.’

There is a sense that Cat has stepped into a fairytale sort of world. Does Althorp feel like ‘home’ yet? ‘I’m not sure it ever will,’ she says, candidly, admitting that Charles can get up to go and make a coffee ‘and then I lose him for an hour’.

‘But these places were not designed as homes. They were made for entertaining and it’s great fun when we fill it with people for something like a literary festival. When it’s just the two of you… well, we try to fill it with dogs and animals.’

The two dogs at her feet seem perfectly at home. Outside, when we have a wander through the stables, she heads up to feed her alpacas. Again, this is not quite where she thought her life would end up.

‘Charles bought them for me for my birthday. It was a surprise. The van pulled up and he opened the doors and four of them got out. I thought, “This is mad. Who gets alpacas for their birthday?”’

Earl Spencer himself – a man who makes a mean scrambled egg for breakfast, apparently – wanders in to say hello as we are doing some photographs.

This is not the same man as the rather portly one in some of the formal portraits on these walls. He’s lost a lot of weight, but there is more to it.

There is a lightness about him in other ways. Friends of the couple have spoken of how Cat – not his usual sort, it is agreed – has been good for him.

She admits that she gave him a shove on the diet front (‘there’s a real history of heart disease and strokes in his family’) and encouraged him to stop drinking, which he has done.

Even she is surprised at how well he has done. He has just celebrated a year of sobriety.

Again, she encouraged him here, a tad shocked, she admits, about how the aristocratic set love a tipple. ‘It’s not that he was an alcoholic as such, but drinking was very much a habit.

‘From that part of society, the culture he grew up in, his friend groups, it was normal to have a glass of wine – or a few – every night. It was a huge part of his life, but he’s got amazing willpower, and he feels and looks so much better for it.’ Cat, too, has a healthier life than she used to.

She does yoga in that Picture Gallery upstairs (‘in Tudor times, it was used for the ladies promenading if it was raining, so they didn’t get their dresses wet’).

And Charles encouraged her to learn to ride. ‘It was terrifying at first, but I’m now at the point where we can go out together.’

She put an extraordinary image of herself on horseback with a bow and arrow on social media recently. ‘That felt like a triumph,’ she says. ‘When I was first diagnosed with MS, I couldn’t write or hold a pen, so it feels like this is a real achievement.’ Medication has helped ‘to hold things at bay’ there, as has simply learning about the condition. ‘Some types are progressive, but mine isn’t and there is no way of telling whether I will ever relapse.’

In a strange sort of way, Karen telling her ‘secret’ was helpful, ultimately. She is now an ambassador for the MS Society. ‘I have that platform, so I realised I should use it.’

What does the future hold? Professionally, she’s shooting lots of arrows. After two successful factual books, she’s just delivered a novel to her agent.

Is it about an academic who ends up becoming a sort of princess, living in a castle? Alas not. It’s about digging up the bones of the past, a crime novel.

Privately, will she and Earl Spencer marry now? It’s another quirk of the British aristocratic system, that while she is, in practice, the chatelaine of Althorp, Karen still has the countess title.

‘Yes, you can choose to keep it,’ says Cat, pointedly. ‘Charles’s first two wives chose not to, so it’s very much a choice. But no, we’re not thinking of marriage. We’re very happy with our life as it is.’

No craving for a title? ‘I already have titles I have earned – my PhD and my professorship. Where would the countess go in that? And I wouldn’t want to give them up. I’ve worked hard for them.’