Downton Abbey is starting to resemble the heyday of The Morecambe and Wise Show. Stars queue up to be in it. Just as Glenda Jackson and Andre Previn seized the chance to cavort with Eric and Ernie, so a Hollywood name like Shirley Maclaine happily trades quips among the teacups with the Granthams.
Now we are about to see one of the greatest sopranos of the past 40 years, Dame Kiri Te Kanawa, in the role of the Australian singer Nellie Melba, singing at a Downton soiree (and raising the incidental question of how the cash-strapped earl could afford Melba's famously enormous fees).
This is a particularly splendid feather in Downton Abbey's cap. What is extraordinary, however, is that the legendary Dame Kiri is quite sincerely ecstatic to be part of the programme. Like 120 million others around the world, the New Zealand-born goddess of the opera house is a bona fide fan.
"I nearly choked when I saw the email that invited me," she says, before describing how, during a stay in New York, she downloaded the previous series then watched it every night in her hotel room.
"I rationed myself to an episode at a time. Once I did watch three at a sitting. They're like chocolates. You try and just have one ..."
We are talking in a suite at the Mayfair Hotel. Smart as paint in her black trousers and red high-collared jacket, Dame Kiri has the courteous, smiling regality of a true star, but also a down-to-earth Antipodean warmth. Writer and critic Bernard Levin, who was so besotted with her that he proclaimed "When I die they will find 'Kiri' written on my heart", also said, more judiciously, that she "carries such conviction because [her performance] comes from a nature in which there is no falseness, no dissembling". I have only seen her sing on film, in Joseph Losey's marvellous 1979 Don Giovanni, and on television at the wedding of the Prince of Wales to Lady Diana Spencer, but it is easy to recognise the truth of Levin's perception.
At almost 70, she remains an astonishingly attractive woman, smaller than one expects and looking 15 years younger than her age. Enthusiasm is a youthful quality, of course, and it bubbles when she talks about the Downton shoot. "I didn't really sleep the night before. It was exciting - and frightening. The first moment of sitting [among] this incredible group of actors - who have been in our sitting room so often ... I remember the cold of the day, the big coats they gave us. And the great hallway at Highclere where I sang - amazing acoustics!"
As Nellie Melba, she gives two of her character's favoured pieces: the Puccini aria O Mio Babbino Caro and Dvorak's Songs My Mother Taught Me. Although the material had been pre-recorded in Vienna, she sang on set - where every cast and crew member congregated to listen - and it is, indeed, this live version that will be heard in the broadcast episode.
What, I ask, is her theory as to the appeal of dear old Downton?
"Because you look back and think, oh, I wish life was still like that. I don't mean the 'upstairs-downstairs' part of it - but when there was gentleness and there was respect. The world is so whizz-bang now.
"I mean, I was at a grand dinner the other night - like a Downton dinner. And everybody there was on their phones ..."
Opera, of course, belongs to that older world; or should do. Careers today can also be rather "whizz-bang", and much of Dame Kiri's time is dedicated to nurturing young singers through the Kiri Te Kanawa Foundation, down a kindlier, more careful path. Earlier this year she also spoke out against the pressure upon singers to look like film stars. She was quite right, although the irony is that her own beauty did not hinder her swift surge to prominence. "One could hardly understand why Don Giovanni's affections should be transferred elsewhere," a critic wrote of her Donna Elvira.
The glory, however, is that her creamy and celestial appearance was like a pictorial expression of her voice. Talent of that kind is destiny. Having spent her earliest years as a popular entertainer in New Zealand, she left to enrol at the London Opera Centre in 1966. "I had a responsibility to my voice - and to my parents."
Born in 1944, of part-Maori ancestry, she was adopted by the Te Kanawas at a young age (her adoptive mother's great-uncle was Sir Arthur Sullivan). "They suffered. They adopted me and they lost me. I thank them for the sacrifices they made - not just financial, but for encouraging me to go - my mother pushed me on to the boat. And I sang my way over. I gave concerts every night for three weeks and got a free ride."
A 1968 review of an Opera Centre performance noted her "rich promise", but, she says now, "I didn't realise what was going to happen until one day at Covent Garden in December 1971, I did the Countess in The Marriage of Figaro. And then the freight train started and it never stopped." The role was, indeed, defining. She "looks and moves like a goddess", it was said, and sang the opening aria, Porgi Amor, "as if its reputation for difficulty were a fairy story".
After this came Marguerite in Faust, Mimi, Desdemona, Fiordiligi in Cosi fan tutte: all given that quality of sublimity that made critics swoon. She was a star, but the royal wedding in 1981 made her a phenomenon. "It was petrifying. You know when you can hear silence? It was this roar of silence - and then me." She recorded West Side Story and South Pacific with Jose Carreras, and her ability to popularise gave her a hugely important role in widening the appeal of opera without - crucially - lowering its exigent standards.
"I care about the future of opera. I want it to survive. And I show that you can have a career for a very long time. You've got to go by certain rules. How many times in the last 40 years did I go and swing at a party? Never!"
Now life may be more relaxed, but it is scarcely less busy. The mother of two adult adopted children, Dame Kiri divides her life between New Zealand and Sussex, and gives concerts to raise money for her foundation. "This is one of the happiest times I've been in - helping young singers," she says. A vigorous, outdoor woman, she loves golf, swimming, and walking the three Yorkie-Pomeranians whose photos we coo over rapturously. "They've been in all the opera houses - they changed the rules to let them into Covent Garden."
So the dogs will be waiting backstage, next year, when she marks her 70th birthday on March 6 at the Royal Opera House, in La Fille du regiment with Juan Diego Florez. The speaking role of the Duchess de Krakenthorp has been expanded a little; indeed the Duchess now gets to sing. How fitting that Dame Kiri Te Kanawa will be heard, once more, in the venue of her first great triumph.
Hit TV series returns to Prime
Prime TV will be playing the fourth season of Downton Abbey - with Dame Kiri
featuring in the third of the season's eight episodes - in the coming months. As a
likely curtain-raiser, the channel is repeating the show's 2012 Christmas Special
next Saturday, October 12, 8.30pm.
Dame Kiri is releasing new album Waiata, a collection of Maori songs recorded with the New Zealand Symphony Orchestra to celebrate the 50th anniversary of her
recording career on November 15.