Forty years ago, one man found the devil in Ms Asher

To anyone who knows Jane Asher as the Queen of Cakes, her performance in Jerzy Skolimowski's Deep End (1970) will come as a revelation.

The wholesome, sensibly glamorous, well-spoken redhead whom TV viewers in their thirties or beyond may remember as the face of McVitie's biscuits or as Jeremy Irons's fussy, ailing wife in Brideshead Revisited, is a long way removed from her role in Skolimowski's soon to be re-released film

Deep End is a swinging 1960s British coming-of-age story with a morbid and darkly erotic undertow. Skolimowski was a Polish poet and former boxer whose career has always (unfairly) been overshadowed by that of his close friend, Roman Polanski (for whom he wrote Knife in the Water).

"We clicked immediately and he offered me the role," Asher recalls of her first meeting with Skolimowski. She relished working with the Polish director. "I knew him mainly as having co-written Knife In The Water, which was one of my favourite films. I didn't know much else about him. He is very charismatic, very attractive and very Slavic."

On set, she relished his eccentricities and his willingness to collaborate. (It was Asher's idea to tear off the head of the famous Saatchi & Saatchi poster of the pregnant man and add in a joke about "defacing public property".)

Skolimowski clearly saw the devil in Asher, even if English directors at the time did not. He didn't want anyone else to play Susan, the femme fatale of the East End swimming baths who leads her adolescent co-worker (John Moulder-Brown) on a merry dance, playing on his strange mix of lust, idealism and naivety.

The Asher who turns up in the bar of a Chelsea hotel on an April morning has just turned 65 and is not remotely devilish, though her hair is as strikingly red as ever. Skolimowski, Asher recalls, had seen her in a television series called Wicked Women, in which, unusually, she played a murderer. The 25-year-old was startled by the screenplay for Deep End, which Skolimowski and his Polish collaborators had written in English – a language they hadn't yet fully mastered. "[The screenplay] was very, very surreal. I wasn't quite sure if it was meant to be quite as surreal as this or was not yet in proper English."

To add to the sense of dislocation and oddity, the London-set drama was made in studios in Munich with a German crew. Skolimowski included various details – for example, gin-and-tonic quaffing policemen – which could almost have come from a Carry On film.

Asher describes Susan as "manipulative, enjoying her power in a very cruel way, using her sexuality to tease someone young and naive ... she's a tease in the worst sense of the word. You can see why this poor young man gets so inflamed and so jealous and so miserable that he does this terrible thing."

Skolimowski told her he had been inspired to write Deep End by a story he had read in a newspaper "about a young man who had killed a girl ... he just got to thinking what would incense a young man so much that he could do something so terrible out of the blue".

Asher's disapproval of Susan is self-evident. In her own personality, she is nothing if not sensible. You can't imagine her spending her evenings in Soho sex clubs and cinemas or tormenting her adolescent admirers just for the sake of it. And yet, she insists "there is more than enough of that inside me. It doesn't take a huge leap of imagination to play that kind of character. I hope I would never be as cruel as that but certainly, when I was little, I can remember at school being in gangs and being pretty unpleasant to other girls."

Yes, Asher acknowledges, Susan is the antithesis to her image today as "Nice Miss Cakey Jane". Throughout her career, she has been the subject of countless articles "about Miss Perfect" and so many other articles saying "shock, horror, she is not Miss Perfect". She claims she "just goes with the flow" and cites the fact that she turned up 10 minutes late for the interview as evidence that she is not as organised as profiles of her claim.

However, moments later, she is describing the business plan of Jane Asher Party Cakes in just the kind of meticulous detail you'd expect from the founder and boss of a company that has been in business for more than 20 years. "We genuinely didn't make a profit for 10 years. It just broke even if we were lucky because it is such a labour-intensive business making those cakes. It's not a business to go into to make money. Mail-order sugar craft – that's what has kept us going."

Yes, she can read spreadsheets as well as bake. You might not be surprised to learn that she is very proficient, too, at Japanese flower-arranging and dressmaking.

Asher's screen career began in the early 1950s when she played a small role in the Ealing classic Mandy, about the plight of a little deaf girl. She was five at the time and has only the haziest memories of working with its director Alexander Mackendrick and its stars Phyllis Calvert and Jack Hawkins. Nor can she remember precisely how she was cast: "I think somebody literally stopped me in the street and said to my mum this little girl will be quite good in the film I am doing."

Her mother was a musician, an oboist in several top orchestras before becoming a professor at the Royal Academy of Music and a teacher at Westminster School after her children were born. Her father, Richard Asher, was a distinguished physician. Her pride in his achievements is evident. Around the time she was appearing in Mandy, he was writing an article in The Lancet that defined and named Munchausen's syndrome.

"It was very typical of him not to call it Asher's syndrome. He was a lovely, funny, brilliant man – and he named it after Baron Munchausen, who invented stories," Asher recalls. "These extraordinary patients make up the most amazing stories about how they happen to have whatever they think they have – which they haven't."

Alongside books like Beautiful Baking, she has also published three well-received novels – The Longing, The Question and Losing It. "Funnily enough, they all have a vaguely medical thread to them which clearly must come from my background."

The Asher family home above her father's surgery in Wimpole Street has its niche in pop history – Paul McCartney briefly lived there during his relationship with Asher in the mid-1960s.

She has made it clear that this episode in her life is out of bounds – and always will be. Check the cuttings, and you will find journalists over the last 30 years all performing the same elaborate little dance with her. They'll find ever more ingenious ways to tiptoe up to the McCartney question and she will politely but firmly sidestep them. Questions about the 1960s in general make her highly suspicious. As she rightly surmises, seemingly innocent inquiries about 1960s pop culture or how the King's Road has changed over the years almost invariably have hidden meanings. She isn't especially forthcoming about her marriage to the cartoonist and illustrator Gerald Scarfe either. They've been together for many years, having met in 1971, and have even collaborated on certain projects (he provided the drawings for her Moppy children's books). However, her pride in her parents is matched by that in her children. Her daughter Katie Scarfe is an actress. Her sons Alex and Rory Scarfe are the pair behind the royal spoof Will and Kate's Big Fat Gypsy Wedding: Photos from Our Big Day, Like. "Any mention you can give it would be much appreciated," she emails after the interview.

As for her own celebrity, that, she says, waxes and wanes. There was a period after Alfie (1966) when she had the chance to go to Hollywood but she turned it down to appear in a revival of Look Back in Anger at the Royal Court in London. She cites appearing in new plays such as Christopher Hampton's The Philanthropist, "working with a living writer on a new piece of work where that character has never existed before," as the most fulfilling experience of her acting career.

One intriguing passion project she is preparing with Hugh Whitemore is Some Rather Special Dinner Parties, a play about cookery writer Elizabeth David. "She was an incredibly irascible, difficult, rude, brilliantly witty, acerbic, very attractive woman – incredibly attractive. She had lover after lover. She rattled round the world, drinking, fornicating, whatever," says Asher of David. The idea is that she will actually cook on stage (health and safety regulations permitting), so that the audience can smell the wafting garlic.

In the 1980s and 1990s, Asher was a near ubiquitous presence on television. She was the face of a long-running biscuit campaign and fulfilled the role of domestic goddess that Nigella and others have usurped today. Asher is less on screen these days. No longer does she get the McVitie's theme tune sung at her when she is out shopping. Nonetheless, she remains busy, acting, fronting charities, running the cake business. She has written those successful novels, so why not an autobiography? "No, never!" she swears. "One thing is that I genuinely don't think I would remember it all."

As for setting the record straight, even that doesn't appeal: "It can stay crooked ...."

Jane Asher and Deep End co-star John Moulder-Brown will be in conversation at a BFI Flipside screening of Deep End at BFI Southbank on 4 May. Deep End is at selected cinemas from 5 May