Although Fawlty Towers was created by John Cleese and Connie Booth in the mid-1970s, Sybil Fawlty was born during the Second World War.

When hostilities broke out, Prunella Scales and her brother were taken by their mother to a village in Devon and when their father came to visit, they ate in the restaurant of a local hotel. A single memory from that meal with her father would inform the way she played Sybil decades later. “I remember the proprietress of the hotel leaning over the back of his chair and saying ‘Do you find it tasty, Major?’”

In six words, Scales brings Sybil Fawlty into her sunny front room; ingratiating, over-familiar, gauche in her eagerness to please. Then as quickly as she appeared, she is gone again. “I think that’s where I got Sybil from originally,” says Scales, staring down shyly at a coffee table that is to be the focus of most of her eye contact over the next hour.

Scales, 77, is one of our most respected actresses, having done much work before and since Fawlty Towers. But there is no doubt that Sybil is what she is most famous for. “Sybil’s not Fawlty in top play” was a typical local paper headline as she toured last month in Gertrude’s Secret (by Benedick West, a cousin of her husband, Timothy West). And people still call her Sybil in the street. “I don’t mind. I’m very proud of it,” she says.

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We are talking because this year is the 30th anniversary of Fawlty Towers. But she is astonished when I remind her of this. “Is it really 30 years? It feels like about five.” She can be a little vague.

Scales was a well-known actress, particularly from the 1960s sitcom Marriage Lines with Richard Briers, when she was suggested for the role of Sybil by the producer John Howard Davies. She had never met Cleese before. “I went to see John. He was in bed with flu. He said: ‘What do you think of the script?’ I said: ‘Absolutely brilliant.’ He asked if I had any questions. ‘Yes. Why did they get married?’ He said: ‘Oh my God, I was afraid you would ask that.’”

They began discussing the characters and Scales worked on the back story. “She was the pro in terms of hotel management and he was the posh one. She had pretensions for poshness. He fell for her because she was attractive in a rather corny way. And she fell for him because he was posh.

“What I thought had happened was that he went to have a drink and she was behind the bar and he thought she was quite sexy and she fell for him because he was so posh. I think her parents had a boarding house and were lower middle class.”

From this Scales worked up Sybil’s grating voice, her ghastly laugh and shrill nagging, along with her interpretation that “she adores him beneath it all.”

The shows were filmed live in front of a studio audience on Sundays. Rehearsals started on the previous Tuesdays. “If you didn’t know it by Thursday morning, there was big trouble.” Cleese is “a very intense person” but, she adds quickly, “a delight. I can’t remember him getting cross.”

One of the striking things about the shows is that most scenes were filmed once without the opportunity to reshoot if things weren’t quite right. On the new set of remastered DVDs, Cleese provides a commentary that reveals him as a frustrated perfectionist. He chuckles along at classic moments — “excuse me laughing at my own jokes, but I think that was Connie’s” — but is also hypercritical. He points out where moments of slapstick weren’t spot-on, analyses how lines could have been tweaked and picks over minute editing decisions: “Could have taken the first three seconds off that shot.”

But what is striking is how quick the show still seems. “It hasn’t lost pace, that’s the great thing about it,” says Scales. “And that’s because of the talent of Connie and John and the rigour of John; how fast he played. If you couldn’t keep up with him you had to really get in there. That’s what its enduring quality is, the pace.”

The reaction of the audiences when they recorded the shows was positive, but she had no inkling that she was involved in something that would go on to top polls of the greatest British television programmes. She sees Cleese, Connie Booth and Andrew Sachs when Cleese is in London. “When he’s over he has dinner parties for all his old colleagues.”

Scales’s mother was an actress, though she stopped working when she had children. Prunella grew up in “darkest Surrey. I can’t say it was remote but there was no gas or electricity.” She acted a lot at school and then after the Old Vic Theatre School, where Joan Plowright was a contemporary, started work straight away and has hardly stopped in almost 60 years.

After Fawlty Towers there were popular comedies such as Mapp & Lucia and After Henry and some films, including A Chorus of Disapproval, but the stage, rather than films, has remained her first love. “As a young actress I remember a producer saying to me: ‘You are a wonderful actress. What are we going to do about your face?’ I didn’t have an Audrey Hepburn face.”

She makes a jolly good monarch, though. She played the Queen in Alan Bennett’s television play A Question of Attribution and has toured her stage show, An Evening With Queen Victoria, all over the world.

She and Timothy West, who is off filming, have enjoyed one of the longest high-profile thespian marriages. They met while in a play when he was married and had “a polo mints and Times crossword flirtation.” When the job ended they started writing to each other.

“I shared a flat with three other girls and he turned up on my doorstep with a tray with a breakfast set on it and said ‘Jackie’s turned me out, she’s having an affair with the lodger.’ And so I and I my three flatmates took him in and it went on from there. He writes brilliant letters. Still does. He sent me one the other day when he was away: ‘Here’s a letter. Remember letters?’” West has published his letters in a book.

They have two sons, including the actor, Sam, who is currently appearing in the acclaimed Enron at the Royal Court . Their other son, Joe, works in IT in France, where he lives with his family. Their grandson, by West’s daughter from his first marriage, lives in the basement of their wisteria-clad house in southwest London.

The secret of their relationship is that keeping busy and working in different places “maintains your relationship at fever pitch.” I had read somewhere that he always likes her to be made-up. “Yes he does. I don’t have to wear it in bed, but I very often don’t take it off.” When they are not working they like to relax on their narrowboat on the Grand Union Canal.

How long exactly have they been married? That vagueness again: “I can’t remember: 44 or 45. How terrible! I must look it up.” She takes down Who’s Who, from a shelf in the book-lined living room: “1963, we were married.”

She has trouble remembering the names of recent plays she has been in and even, halfway through the interview, which newspaper she is talking to, but says she has no problem learning lines. “People will tell you that learning the lines gets more difficult but actually that’s been the same from the beginning.”

Her appetite for work remains undimmed and she is a lively host. As the hour approaches noon she declares it is “nearly time for a drink, an actual drink” and seems disappointed when her interviewer, the photographer and the PR all boringly decline the offer of a glass of wine.

“I want to die on the eighth curtain call. I want to go on working till the day I drop. The fact that there are the curtain calls means it has been a success. Just hope I am somewhere near the middle and reasonably good in the part.”

The next day, the first paragraph of the obituary will undoubtedly contain the word “Sybil”. And she will for ever be remembered for her wonderful delivery of a single word, freighted with spectacular bossiness: “Basil!”

Fawlty Towers Remastered, a boxed set of three DVDs, is released today

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