Her own outlook is eclectic. She is broadly proud of Britain’s empire, the subject of her three-volume history Pax Britannica. ("I met so many people who devoted their lives to it without any thought of being bosses or racists or anything.")
But she is also an enthusiast for modernity. Cruise ships in Venice? "The doges would have loved them! Showy, moneymaking, marvellous engineering!" Double-decker tourist buses? She often took them: "For an introduction to the city, what could be better than that? Don’t be stuck up about it."
For Thesiger, the prospect of space travel could not have been less appealing. For Morris, the opposite. Indeed, after partially scaling Everest, she expected Nasa to call on her to perform a similar role for the space expeditions. "I thought I was the obvious person to go." Reality quickly struck.
For these reasons, and because she loves fast cars, Morris is fixated on Elon Musk, the SpaceX founder who wants to colonise Mars. "He’s the most interesting man alive, I think," she says. "What’s his background? He’s got a family… He also hasn’t joined the celebrity circuit, has he?"
The food has arrived, and Morris wrestles to contain the fish and avocado in a flour tortilla. After her sex change, she noted that waitresses started mothering her, and sure enough an extra plate has been fetched. Meanwhile, I am rediscovering that mussels can have a mysterious crunch.
Morris says she has written "solid books" about only five cities: Trieste, Oxford, New York, Hong Kong and Venice; the rest of her work she dismisses as "flibbertigibbets".
Her talent is to give places personalities, to generalise about their attributes. Trieste had "the flavour of true civility". Oxford was "almost a civilisation", where there was no norm and where no one was entirely wrong; Manhattan was a surprisingly "human city, where personal aspirations, for better or worse, unexpectedly take priority".
But her finest observations were perhaps about Venice. "She fitted into no convenient category of nations. She was the lion who walked by herself," Morris wrote of the city in 1960, appearing to recognise a kindred spirit. Venice was no longer the meeting point of east and west by then, but "she only awaits a summons". Venetians, meanwhile, were provincial, curious, meditative.
I ask how this fondness for generalisation fits with Morris’s own unique story. "Don’t understand the question." Well, what I mean is, if we see the world through generalisations, we would never conceive of an individual story like her own.
"I know what you are, poor fellow, worrying your way around, and that is the matter of sex." I protest, but we both know that on the notepad in front of me are the words sex change in capital letters. "Don’t worry, I’m armed," Morris says.
Did it change your writing? "Not in the slightest. It changed me far less than I thought it had."
What became of her surgeon in Casablanca? "I think he died, but I’m not sure." Did you stay in touch? "No." (Dr Georges Burou drowned in Morocco in 1987.) Did the sex change overshadow your books? "It did at the beginning, of course… But it’s faded now."
Does she consider whether she would have achieved more as a man? "No." Her account does not always match up with what she wrote in Conundrum in 1974, but why should it?
There are no regrets, but it strikes me that Morris is without a cheerleader. In her own words, she is "always an outsider", a "loner". She is not part of any literary set, and during our conversation barely mentions any friends outside of this corner of Wales. She is a pioneer with little interest in the following pack.
Why hasn’t she been made a dame?
"They’ve given me a CBE [in 1999]. I consulted Plaid Cymru [the Welsh nationalist party of which she is a member] before I accepted it. They said, ‘Go ahead — it wasn’t for you, it was for Wales’."
I push aside my plate, and vow not to eat mussels again. "That’s the stage where I would pick up the bowl and drink out of it," says Morris, smiling.
These days travel has become "very laborious", but "if someone rang and said, ‘Would you like to come over to Manhattan to take in some function?’, I’d go."
Is there anywhere that she regrets not visiting? "Oh, Lhasa," she exhales immediately. "I always said I’d never go until the Dalai Lama went back."
Even at a distance, she retains her grasp on the world. "The one thing people felt about Brexit was too many bloody foreigners coming in and changing the country. And it’s impossible not to feel some sympathy for them, isn’t it? Let’s be honest with ourselves."
She is hopeful that the Welsh are generous enough to integrate newcomers, as they have in the past. "I know that’s a rash generalisation, but I believe it to be true." She pauses. "Of course there are shits as well."
Yet Morris’s books assumed that places determined people’s identity; perhaps, the more people migrate, the less true that link is. "I agree," she says.
Morris orders a flat white. Once the waitress has left, Morris turns to me, with an alarmed expression: "Excuse me, I don’t know what a flat white is." Ah, I think, she’s not as with it as she seems. Do you want me to cancel, I begin. "No," she interrupts. "I love it but I don’t know what it is." I confess that I don’t know what a flat white is, either.
We stray back on to the sex change. "I think of myself as both." Both what? "Both man and woman. Or a mixture of both… I thought I was going to be distinctly on one side. I realise now, not so. I’m both."
Feminists, notably Germaine Greer, criticised her for simplifying gender traits — for assuming that men were obsessed with sex and power, and women were emotional "saints". "Yes, and I think I did [simplify] to begin with. But I have different feelings about it now."
Why? "Dunno. I matured, I suppose."
Morris has just passed the point where she has spent more time in a woman’s body than a man’s. She is truly two of a kind. "Looking back on my life, of course I had this feeling that I was in the wrong sex and I had to get out of it. But it didn’t occur to me then that the ultimate object might be to be both. And the next object is to be neither."
Ah, yes — that’s what people talk about now, a world beyond gender roles. "I was thinking of death," she laughs. "Not many stages in between now, are there? Only the withdrawal of my driving licence."
She is ready for the day: she’s even written a book on allegories, to be published posthumously. Why not until then? "I don’t really know," she says. "We’re stuck with it now."
We continue, talking of whether Britain can afford two aircraft carriers, how she turned down the chance to be a TV presenter, and how she wanted to found a society for kindness, "a sort of political party".
"I think kindness is the answer to all our problems," she says. "Is Mr Musk kind?"
Eventually, I order the bill and a taxi. Morris relaxes. "I have to admit that I enjoyed our session," she says. "I was dreading it."
Dreading what? "You. What I was really dreading was you’d get around to this bloody business of sex change as the centrepoint of our conversation. Which it didn’t become, did it?"
But she wrote that the change was the climax of her life. "It was then, wasn’t it — of course. That was 50 years ago."
Finally I realise. Age sometimes wearies Jan Morris, but it also puts her most extraordinary life in the context it deserves. Longevity does have its consolations.