Being 80 means ditching the heels, putting your feet up and knitting for the grandchildren - try telling that to Joan Collins! As her new book, Passion For Life, is published, the actress tells Sophie Herdman why she loves being busy, hates advice-dishers and will never stop being an optimist.

The phone rings and a man picks up - unless she has just eaten a Snickers bar, this is clearly not Joan Collins.

In fact, it's Percy Gibson. Husband number five. He passes her the phone.

"I'm getting a bit of a sore throat so forgive me for being a little croaky," she says.

As ever, Collins is extremely busy, and she's been struggling to sleep due to the building work going on at her London flat.

"It's hard to find a little nook that one can go into without bumping into someone," she notes.

Among her current projects, there's a new autobiography Passion For Life, two films, a cameo in TV series Benidorm, a novel, a big secret project (you'd never guess this woman is 80) and she's due to perform an updated version of her one-woman show next year, which Gibson directs.

He is wonderful to work with, she says. And according to her new autobiography (or illustrated memoir, as she'd rather call it), he's also a wonderful husband.

So what does Collins, with her wealth of experience, believe is the key to a happy marriage?

"Giving each other space and being best friends," she states. "Oh - and trying to have separate bathrooms."

Collins is not really one for giving advice, though, and she certainly does not tell her three children how to conduct their relationships.

"If someone asks what I think of their boyfriend, I will say, but I don't arbitrarily go about giving advice. I hate people who do that," she says.

When she wants advice, she turns to her husband, and occasionally her younger sister, writer Jackie Collins - but only "up to a point".

Family is clearly very important to her. The first chapter of Passion For Life is dedicated to the topic (there's another about her many friends - loyalty, she says, is key to a good friendship).

Collins's mother was a dance teacher, her father an agent for the likes of Shirley Bassey and The Beatles - though she describes him as unemotional.

"He wanted me to go to secretarial school, find a good husband, have children and lead a nice, proper life.

"It wasn't quite my cup of tea," she says wryly.

He told her she'd be washed up by 23 if she became an actress.

"That was rather prevalent then - there was a tremendous amount of ageism with actresses, and I think there still is."

His comments just made her more determined to succeed.

"I took some small pleasure in showing him that it didn't happen the way he thought it would - that I was able to make a living as an actress, which very few actors can do all their life, and write books and support three children," she says.

"I feel I've achieved quite an amount in my time."

Her father did give her one good piece of advice, though: No one will ever do anything for you, you have to do it yourself.

"He was right there," she says. "I trusted my financial affairs to people who were supposed to be experts during the time of Dynasty, and they badly let me down. I found myself in very deep trouble with the tax man."

That's not the only knock-back the actress has experienced during her life.

There were four failed marriages, of course, starting with Irish actor Maxwell Reed (whom she married aged 19; it lasted four years), followed by another actor, Anthony Newley, who was unfaithful. Next came American businessman Ron Kass, who developed a drug addiction, and finally pop singer and playboy Peter Holm, whom she describes in her book as "a mixture of obdurate dullard and calculating sociopath". This was the shortest marriage, lasting barely two years.

Collins also suffered heartache when her daughter Katyana, then aged eight, developed a severe brain trauma and was left fighting for her life in a coma following a car accident. Thankfully, she lived to tell the tale.

The actress was once also embroiled in a high-profile legal battle with publishers Random House.

Somehow, though, she seems to have remained positive.

"I was born with the optimist gene," she says.

High points have been plentiful. She started working professionally in her teens and was quickly signed to Twentieth Century Fox, landing her big Hollywood break with a role in 1955's Land Of The Pharaohs.

Film and TV roles continued throughout the Fifties, Sixties and Seventies, including two films based on novels by her sister - The Stud and The Bitch.

From 1981 to 1989, she starred as scheming diva Alexis in Dynasty, which became one of America's most successful series and earned Collins a Golden Globe.

She's continued working in film and on stage, has also written a substantial number of books, both fiction and non-fiction, and right now, of course, she's popping up in the aforementioned Snickers adverts.

In 1997, Collins was awarded an OBE for her contribution to the arts and charity work. She's a big fan of the Queen.

"I think she is the most inspirational woman - becoming queen at the age of 25, she has never put a foot wrong. I think she's absolutely brilliant," she notes.

Margaret Thatcher's another woman she admires. "I like women who have a strong sense of their own identity, who don't necessarily conform to a standard that other people expect."

Her first four marriages may not have lasted, but Collins doesn't see them (well, two of them at least) as complete failures. Without them, she wouldn't have had her three children - Tara and Alexander, whom she had with Newley, and Katyana, by Kass.

She's looking forward to spending Christmas with them.

"I'm a huge fan of Christmas! Every year, I have a massive tree. I started making my own decorations when the children were small - they're almost antiques now - so we decorate the tree and the living room," she says.

Her three grandchildren will be there, too.

"I enjoy my grandchildren, but do I enjoy being a grandmother? What does that mean? Sitting around making woollen hats for them?" she says, laughing.

Indeed, it's hard to imagine an 80-year-old, who can still do the splits and effortlessly pull off a bodycon dress, doing anything traditionally 'granny-ish'.

"I think there's far too much emphasis on age these days," she states. "It's becoming ludicrous - everyone's obsessed with everybody else.

"I don't think one should be defined by age," she adds. "There are people of 50 who look like crones and people of 70 who look brilliant."

Collins is all about the glamour, but she's opted for leggings and a striped jumper today: "Because I'm just lying around the house."

Lying around and speaking to journalists, it should be noted, so it's hardly a day off.

But Collins is not really one for slacking and admits she has no plans to slow down.

"I don't think I'll ever do that. I'd be bored stiff. I can't think of anything more horrendous than getting up and the only thing I have to look forward to is watching TV. No, I'll always be doing something," she says.

"You know what they say: 'You've got to eat life, or life will eat you'."

:: Passion For Life is published by Constable, priced £25. Available now