Celia Birtwell, a name that once seemed firmly consigned to the past, is enjoying a prodigious renaissance – and her new fans, legions of them judging by her sell-out collections, are the daughters and granddaughters of the generation of women in the late Sixties and early Seventies who once wore, or could only dream of wearing, those gorgeous epoch-defining frocks, the fabric designed by herself and tailored by her ex-husband, the late, murdered Ossie Clark.

It’s rather marvellous to think of 16-year-old schoolgirls stampeding Topshop – where Birtwell’s limited editions tend to sell out quicker even than the Kate Moss range, in minutes rather than a day – to buy floaty mini and maxi dresses in the very same prints, the styles only slighty updated from the originals, which were worn by the likes of Bianca Jagger and Marianne Faithfull in their own dewy youth.

This summer marked Birtwell’s fourth season with the high-street chain since her debut collection in 2006, and she was chuffed and amused to have a window devoted to herself in Oxford Circus, “With all my daft little girls and skirts that come down just past your knickers and banners – a bit like a carnival. It’s called ‘Young and Cool’, I think.”

There was another collaboration, with Millets – perhaps even more unlikely, given that “happy camper” is not a phrase one would readily associate with Birtwell, certainly not in the outdoors sense – in the spring, featuring tents and wellies, sleeping bags and golf umbrellas, all looking weirdly desirable in subtle colours and delicate patterns. There was her cover of an Elizabeth Taylor novel for the 30th anniversary of Virago Modern Classics earlier in the year, a forthcoming BBC Two documentary made by the same team behind Andrew Marr’s History of Modern Britain which charts “Celia’s survival”, as her assistant puts it, from the late Sixties to her current rebirth, and her contribution – a homage to Manolo Blahnik, who designed shoes for Clark and Birtwell’s runway shows – in the recent 20th anniversary issue of Marie Claire magazine.

More licensing deals are in the pipeline, the Celia Birtwell range of “girlie toiletries” for Boots, wash bags, and little bags for brushes and emery boards and eyelash curlers – “Not that I’ve ever known how to use them” – and sunglasses, engraved with Birtwell’s distinctive flowing signature and various designs from cat faces to stars or stripes in charcoal and powder blue. She agrees that the Birtwell brand seems to be everywhere at the moment, and says that a lot of it’s down to Antonia, her publicist, and her daughter-in-law, Bella, who decided that, “Celia Birtwell ought to be licensed while she’s become something in her older life.”

But Birtwell has her own secret (not for much longer) agenda, which emerged when the Queen of Prints said that she hasn’t worn prints herself for a good few years. Isn’t this admission a bit close to “doing a Ratner”? She laughs, and it’s a surprisingly dirty chuckle escaping from that rose-red cupid’s bow.

Despite her doll-like demeanour – she still looks strikingly like the young woman in one of Tate Britain’s most popular paintings by her old friend David Hockney, Mr and Mrs Clark and Percy (1970-71, reproduced above), the same limpid gaze and topknot of cherubic curls – there’s nothing mimsy-pimsy about Birtwell. She has that same no-nonsense strand shared by other creative northern types of her generation: Sir Ian McKellen, Hockney and even the bossy former schoolteacher occasionally glimpsed from under Vivienne Westwood’s veil of eccentricity.

When I ask Birtwell whether she has a beauty regimen, for instance, she says: “I think we’re told an awful lot of rubbish about make-up, actually. Do you remember that Boots thing [they are selling her products, remember], when they all went bonkers and sold out of this stuff… I mean, if it’s really serious, it would have to be on prescription. It can’t work. It’s not going to, you just believe it does. Anyway, I’ve always used soap and water on my face.”

How does she feel about Botox? “No, no, no – I think that’s far too vain. I would not do that. I feel there’s a conscience within me in a way, and I’m also quite a coward. I wouldn’t do it because, when one reasons it out, how fortunate are we to get to my age and actually be well? And I think, in the end, you really shouldn’t do it. I don’t really agree with it.” You think it’s a bit immoral? “I do really, yes.”

Warming to her theme, she continues: “This whole business of vanity… You know, what is beauty? We always reflect so much on youth and then it all disappears. You just live, don’t you, after a certain age? You’re just a being. And we don’t really have many icons that are older. Very few. And you see people of my age, I’m afraid, and they’re probably quite a bit younger than me, but what do they look like?

“The ones who haven’t got any style at all and they’re all out there, aren’t they? I think it’s rather sad, actually, when you look around. I sometimes think the Queen is quite an advocate of how those women look – when you see the grey hair curled up and those clothes… I don’t know who they want to look like or… I can’t get to the bottom of that one.”

Birtwell certainly manages to look stylish, despite her own shopping frustrations. She is all in black, an Agnès B chiffon blouse, tailored trousers, “a very old” pair of Charles Jourdan shoes with a delicate heel, a Topshop tasselled bag for her mobile phone, and a splash of red in her own Celia Birtwell scarf.

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