Jilly Cooper is giving an awe-struck viewing of the Mandarin Oriental's Royal suite, a muted monument to scale and privilege. "Wonderful, wonderful, wonderful . . . I couldn't find the telephone . . . Come and have a look. Look at this bit here," she gurgles, as her PR woman mutters something about the doors being locked at four. Ours is the final interview in the suite. We marvel at a vast bathroom, the steaming sauna she will never use, the exquisitely-laid dining table off the wraparound terrace overlooking Hyde Park.

Ah here, surely a woman who has spent the bulk of her life idolising, observing and mixing with British high society must be well-acquainted with such opulence? “No, no, I don’t. I’ve been sitting in the country that long.”

And she just wants to be back there in the country with Bluebell, her rescue Irish greyhound, with whom she has an "over-close relationship"; back to the 700-year-old Cotswold-stone pile in Gloucestershire, crammed with animal paintings and photographs. "I can't say I don't love my dogs better than anybody, frankly," she says. Which is almost certainly true. Camilla Long, her Sunday Times interviewer, counted at least 70 animal knick-knacks in the sitting room alone, " . . . dogs, dogs, dogs – open any cupboard and a dog corpse could fall on top of you". A glib comment which wounded Cooper to the quick.

It’s difficult not to feel a twinge of guilt about being in the queue to knock a louche tale or two out of Jilly. Maybe it’s a generational thing. Younger readers may not know or care about the 79-year-old with the blunt mats of blonde hair; this tiny, tired and vulnerable-looking woman in her oversized silk shirt, promising lots of “human nookie” in her latest, horsey shagbuster .

But many older folk remember a ground-breaker, vibrant, fearless, funny and confessional, who landed her first column as a young bride in 1969, after regaling a Sunday Times editor with stories of exhaustion from rampant sex and housework. After losing the first draft of Riders – the first instalment of her dirty, silly, Rutshire romps – on the bus around 1971, she finally introduced Rupert Campbell-Black, Rutshire's swaggering, showjumping, posh boy, to the world in 1985.

Back then, there was just her and Jackie Collins offering bonkbusters. But Riders also offered the bonus of sharp social observation of typical British types and all classes lapped it up. With the royalties, she saved the country house from the nasty bank and the family from (relative) penury, enabling her "lovely, lovely, lovely" husband Leo to carry on with his worthy military books publishing, and giving employment to legions of staff. For all her apparent skittishness, she has sold 12 million books. Her work ethic and prodigious output have never faltered in nearly 60 years. Respect.

In fact, she is in the throes of her next doorstop. This one is about football, titled Tackle! (probably because Score! was already taken by one set in the opera world), in which the club owner is a vegetarian who feeds the team vegetarian pies at half time – "which are delicious".

She continues to write not for distraction but because she needs the money, she insists. “I live in the most enormous house, a lovely house in the country and also in six years I haven’t had an income except a bit of royalties. I need the money because I have children and a lot of people working for me and we had carers for Leo, two at a time . . .”

It must be a hard act to pull off, slogging over a 600-page shagbuster then providing the required fruity quips and outrageous opinions, following a stroke, a hip operation and Leo's lingering death after a 13-year illness with Parkinson's. It took her six years to write Mount! – set in the flat-racing world – because Leo died half way through it.

But if, having sustained a 50-year-marriage, infertility, his crucifying infidelity and cruel death, she expected a sympathetic ride with Mount!, she was mistaken. Long's take on Leo was swingeing, reminding readers of his affair in London while his wife was sweating away on Riders, characterising him as a "dud", "bossy and dogmatic", and "a dillentantish second-rater" who liked to tell people how little of her stuff he'd read while relishing the cash and lifestyle it generated.

She crumples a little and waves her hands as though trying to push it all away. “Oh don’t . . . it was awful. She was horrible about my darling. She was foul about him. And I don’t know why . . .”

Briefly, the Jilly who is relentlessly positive makes a comeback: “I think she was trying to be sympathetic to me and to present a different take on it. I think in a nice way she was trying to think, ‘poor Jilly, she’s had a difficult life, she’s had to work so hard to support all those people’.”

Then she gets sad again: "She obviously took against Leo, which is a shame because if she'd met him, she'd have loved him. It upset me terribly that piece, simply because he wasn't like that. He was a man of substance, publishing wonderful books. On his grave, it says 'Leo Cooper, great publisher who gave soldiers a voice'. And he did. Before he started publishing, there were lots of military books, but they were academic stuff, colonels writing, but he got all the soldiers and published all those wonderful books – eight volumes on the history of the British Cavalry by the Marquess of Anglesey. To say he was a failure and a dud, well, that was awful, just awful."

And yet, Jilly's own journalism was hardly a thing of delicacy. There was Princess Michael of Kent for one, a "great friend" who hasn't spoken to her for 30 years since Cooper wrote a piece which left her forever dubbed "The Pushy Princess".

And Theresa May is doubtless delighted with such Jilly dismissals (in the Sunday Times piece) as being "dingy", given a soft ride "because she's a woman", one who "hoisted her skirts up . . . plus boobs", while campaigning. All followed inevitably by the lament against political correctness: "You can't say anything now . . ."

Of course, it's all shriekingly good fun until the PC deficit hops on you, one expects. Might this be a touch of karma come to bite her? "No, I don't think that at all," she says in slightly clipped tones. "All I minded about was she was being horrible about my husband – I don't care about that. Obviously we've all done awful things. When I was in the Sunday Times, as I've said, I was so pleased to have a column I was like a child with a brick on his rope; I didn't mind whose ankles I hit. I'm amazed how fearless I was in those days. And . . . maybe you're right, maybe it was karma. But I just think it was wrong." She sighs indignantly. "I mean 'you open a cupboard and a dog's corpse will fall out'? – as if I'd let a dog's corpse fall out of a cupboard. They're all buried in the garden."

She brightens at the memory that Long "did" Mary Berry from The Great British Bake Off in the same week, "which I thought was good because it was obviously geriatric-bashing day . . . She said Mary was very snobbish because she curled her lip at Paul Whatsisname dunking his chocolate biscuit."

She has a theory today that no one really reads this stuff anyway, that “the general public probably reads the first two paragraphs and they see a beautiful picture and think ‘lucky old Jilly’ and move on to the next page.” They might. Though gallantly, she notes several times that Long is a very good writer and “such good fun”. So then again, they might not.

Cooper had been photographed with a beautiful rescue grey stallion for the piece. It had also made an appearance at her launch party in the Mandarin Oriental the night before our interview, nibbling grass in the hotel garden and being fed Polo mints by guests. In a brilliant publicity wheeze, Cooper's purported real-life inspirations for Rupert Campbell-Black – a trio rejoicing in such names as the Earl Mickey] of Suffolk, Rupert Lycett-Green and Andrew Parker Bowles – also made a jolly appearance. If these were her role models, some or all of them must have been pretty awful back in the day, since she agrees that her adored hero, Rupert, was a "complete shit" in Riders. "But he gets much better in Mount! – much tenderer and sweeter." Which may or may not describe the three lads' life progression too.

Her more out-there opinions are often revised, some in the space of a few weeks. She thinks Theresa May, for example, “is coming along quite a treat now . . . She looks good, doesn’t she? And she looks quite strong, though it’s very early days.” But you said she got a soft ride, hoisted up her skirts and stuff like that? “Emm, that was a bit . . . But it was such a long interview,” she says, wondering plaintively why there can’t be more nuance. “It’s very easy if you’re having a lovely long chat with somebody to sort of nuance it, isn’t it?”

Did she really love being groped as a young woman, as she claimed? “I feel awful about this. You have to realise that somebody making a pass at you had to be attractive. Obviously an attractive man, you want them to make a pass at you, don’t you? But now I think, even if somebody attractive makes a pass at you, lots of women say ‘go away’, don’t they?,” she says, sounding baffled.

At times, she suggests that some of the more head-spinning quotes come from old columns, from another era, when “of course it was a different time” and her “brilliant” editor “made me write all sorts of things . . .”

This leads to an awkward exchange about elderly men such as Rolf Harris now serving time for assaulting young women in the good old days. Harris, she says, "was a friend, a great friend. Obviously people do things . . . I loved him. I'm terribly sorry at everything that's happened. He painted a portrait of me on a programme. Lovely picture. And he did a lovely picture of the Queen." She draws in men such as Cliff Richard and Lord Bramall, against whom allegations ran into the sand: "It just seems a pity that for £100,000, an awful lot of girls can say 'he groped me back in the '70s or something'. Tricky one. It could easily not be true, couldn't it?"

She remembers that era as a time of endless fun. "Everybody was having sex with everybody, I think. It was such a fun time." She recalls a party thrown by Harold Evans, the Sunday Times editor, at which she played ping pong with Melvyn Bragg [the English broadcaster and author], "in a see-through dress with not a thing underneath. Can you imagine the wobbling? I remember Felix [her son] saying 'oh mum, you can't go out in that dress, everybody will see your tits'. He was about five. Su-weet. Leo loved that dress. No one wore bras then ..."

Was she included in the "everybody" having sex? "Oh people used to come up to Leo and me and say 'will you come to bed with us?' And so we used to say no, but there was an awful lot going on." The Polaroid camera had just been discovered and "that's what got everybody going", she thinks, "everybody taking photographs of each other without any clothes on." Which only proves there is little new under the sun.

As for now, and the politics of Brexit, she finds it all "rive-ta-taa". That's Jilly Cooper for "riveting". She voted out, she says, because she did a piece on the "Common Market" many years ago – back when carbon copies were in vogue – and was horrified by the bureaucracy and "lack of freedom". Even so, she was shocked when she woke up next day. "I said 'Jesus what have we done?' – I didn't think in a million years it was going to happen." She loves Boris Johnson and is good friends with his sister, Rachel, so we leave it there.

Still, she seems genuinely concerned about the restoration of a border in Ireland and clearly has great affection for the place.

The book's cast of characters (including many animals who also star) runs to 16 pages, as do the acknowledgments, crushed under the weight of superlatives. They include a page on her stay in Ireland, where she was taken to an eventing weekend and a day at the Curragh, along with visits to Coolmore – "where no petals of the divine gardens were out of place" and where she got to "shake hooves" with Galileo and Camelot; then to Ballydoyle and on to Willie Mullins' yard, where she was charmed to hear the bantam hens have Christian names. Sadly, none of it made it into Mount!. "It just didn't fit".

The tsunami of adjectives comes to a close with tributes to her son, daughter and step-daughter, and a reference to the last three “tough” years, during which she lost not only Leo, but Feather, her black rescue greyhound, and “equally beloved rescued cat”, Feral. The final words are for Leo: “I’d like to thank Leo, wherever he is now, for 52 never dull years. I was so lucky to be married to a great publisher, whose kindness, stoicism and humour in the face of illness never failed him.”

Mount! is published by Bantam Press, priced £13.99