Sian Phillips is very excited. Shirley Bassey was in the audience last night, as was Madonna, and they both came backstage to congratulate her. "Madonna sat on my bed - my bed! - in my dressing room! NO, I can't remember a word of what she said. I was too beside myself. I adore Madonna. If anyone living today is like Marlene Dietrich then it's Madonna, isn't it? She's so charismatic. Ah, now I remember. She wanted to see my dress. 'Show me the dress,' she said. But I couldn't because it had already gone into the safe. Madonna! In my dressing room!" Poor old Shirl. I don't think she got a look-in.

Whatever, I am happy for Sian because, let's face it, it's about time she had a big hit. And Marlene, her one-woman tribute to Dietrich at the Lyric, Shaftesbury Avenue, is proving very much that.

No, the script isn't up to much, frankly. As homages go, it's a lazy one. What you know going in is also what you know coming out. But Sian's performance is dazzling. If nothing else, try to get in for the last half- hour when she puts on The Dress - a Jean-Louis imitation which took "four people seven weeks to bead and cost God knows how much" - and sings "Falling In Love Again". It's worth it just for that.

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The funny thing about Sian Phillips, now I think about it, is that she's always been up there without ever being up there. We kind of know she is a tip-top actress, but at the same time for every Marlene (or Livia in I Claudius) there seems to have been 10 years of doing creaky productions in rep with co-stars who had nowhere to go once their telly shows were up. A touring production of The Lion In Winter, with David McCallum playing Henry II to her Eleanor of Aquitaine, springs nastily to mind.

How, I naturally wonder, has this peculiar dichotomy come about? I mean, you never find Judi Dench or Maggie Smith giving it their all in the sticks somewhere, do you? Is it a problem of range, perhaps? No, she thinks it's to do with "marrying O'Toole and losing the moment". His ego ate hers up, then? Yes, she says, that's about it. And, yes, it's taken her many years to recover. But she insists she isn't bitter. In fact, she's learnt a lot doing ghastly things in rep. "I've learnt," she says, "to love acting for what it is rather than for what it can bring you." Although, if it brings you Madonna, that's OK, too.

Sian Phillips must be 62 or thereabouts - no, she won't tell you her exact age - but she is still quite something to look at. She has those big blue-grey eyes. Her skin is still in good nick. And those cheekbones! They are so astonishing she looks as if she's had her insides sucked out at some point.

Yes, Marlene had amazing cheekbones too, but she cheated by "having her back teeth removed". Also, she kept her skin tight-looking by plaiting her hair and attaching the braids to surgical needles which she then embedded in her scalp under her wig. Yeuch, I say. But Sian says it's simply how women gave themselves facelifts in those days.

"Margaret Lockwood always used to do it. Once, one side fell down while she was queuing for lunch in the BBC canteen. She then had to sit and eat with half a tight face and the other half flopping all over the place. But she didn't mind. She was a great sport." No, Sian has never resorted to such measures. Neither has she had plastic surgery. Yet. "I am not against it. I'm all for it, in fact. I've just read about laser surgery, which sounds wonderful. I shall investigate, believe me."

Sian lives in Kensington, west London, in a two-bedroom flat which has a pretty, sunken garden out back. Here, we sit under a green-and-white striped canvas thingy which looks like a cross between a jousting tent and a chuppah, but which, according to my Argos catalogue, is actually "a luxury garden canopy". Sian calls it her Dame Judi Dench Memorial Canopy because "Judi made me buy it. I'd been thinking of getting one for ages, so she said: 'Do it. Pick up the phone and order it now.' So I did."

Sian lives with two Burmese cats, Rupert and Barnaby, who are company enough for her, thanks very much. Marriage may have worked for some people, but never for her. "All that stuff about what's for supper. Such a waste of time. Sometimes you only want a cup of soup and to get on with your phone calls." Peter was exhilarating, yes, but he also very nearly destroyed her. As for Robin Sachs, her subsequent husband, he ran off with someone else. Men have proved disappointing all round. She likes her cats better.

A few years back, she says, she finally passed her driving test by imagining Barnaby was in the back. "Barnaby's quite a nervous cat and I wanted to give him the nicest ride possible." When she goes abroad, she writes them letters which she carries about tucked in her shirt pocket for a day ("to pick up my scent") before sending them home. "They get very excited and rip open the envelope." She has been known to leave them messages on the answering machine.

Later, when she talks about having had a four-month course of CAT therapy, there is a delicious moment of confusion when I assume that this must be some kind of treatment, possibly invented by Carla Lane, which allows you to relate better to the feline members of your household.

I even, I'm afraid, ask if she took her cats along to the sessions. Sian looks at me perplexed, then, the penny dropping, she good-naturedly explains it's actually something called Cognitive Assertiveness Training. This, she says, enables you to "stop being a wimp" while "getting what you want from life".

I tell her she has never struck me as a wimp. In fact, I add, I've always thought of her as the opposite, as quite a highly strung, demanding grande dame. Sian laughs heartily, then says I'm woefully wide of the mark. O'Toole didn't want her to work, so she didn't. She married Robin to please him - "He was very persistent and I couldn't be bothered not to marry him in the end."

She bought her luxury garden canopy because Dame Judi ordered her to. Not that she regrets it. "It's waterproof and can stay out all year, although I do take it down in the winter."

The therapy, she adds, has helped enormously. She can get to the supermarket check-out and say: "Hang on. Is that French mustard? I meant to get German. Please could you get someone to change it for me?" She can do this even though, she exclaims proudly, "there are 20 people in the queue behind me". And these things have changed her life? Yes, absolutely, she insists. "I've learnt I can disagree with people. I've learnt I don't always have to be adorable."

As a child, she always felt she did have to be adorable. An older sister having died before she was born, and no siblings arriving afterwards, she knew she meant everything to her parents.

Her mother, Sally, was a teacher who had to give up teaching when she married, because you weren't allowed to be a married teacher in Wales in those days. Her father, David, was a trained operatic tenor who had to give up all thoughts of a professional singing career when his father, a miner, died of silicosis, and he had to get a job in a steelworks to support the family.

Both frustrated, they pinned their hopes on their only child. In particular, her mother very much wanted Sian to have a good career. "She actually warned me against marriage. She would say: 'Sian, if you want a career, have a career, but don't try to have anything else.' She adored O'Toole, but still she was rather pleased when I left him. She said: 'Now you have another chance. Don't marry again.' " When Sian did, she refused to attend the wedding.

Sian grew up in a remote Welsh stone farmhouse near the Black Mountains, where her family had been hill farmers for generations. It was a plain- living, high-thinking sort of household and she was precociously clever. She had read all of Dickens, Trollope and Shakespeare well before she was 10. When she sat her 11-plus, she got 0 per cent for maths but 100 per cent for English. "I wrote about usury in medieval times, with reference to The Merchant of Venice." She got into grammar school on the strength of it.

However, she'd wanted to be an actress since she was six and her grandmother took her to a pantomime in Swansea. "It was the first time either of us had been to the theatre and we thought it wonderful. My grandmother came out and said: 'Didn't the girls have wonderful complexions?' She didn't know make-up even existed."

At 17 she came to London, to attend Rada, and a couple of years later met O'Toole. She took him back to Wales, where he was a great success. "He was like an exotic bird that had landed. Everyone adored him. They'd never seen anyone drink like him. The men would stay up late and chat to him. In the morning they'd still all have to get up at 5am and go to work, but he would be in bed till lunchtime. They'd never known anyone stay in bed after 7am." However, he did upset her grandmother, who was responsible for cleaning the shoes for the entire farm. "She was terribly upset because his boots were suede and she'd never seen suede. She kept peering at the boots and polishing but couldn't get a shine."

After their marriage in 1959, they moved to the west of Ireland, where they built themselves a rollicking big house. Here, Sian had the babies - two daughters, Kate and Pat - while Peter went off to do rather more glamorous things, like Lawrence of Arabia. Yes, she was lonely and felt isolated. Being able to work would have helped, but Peter was against it. "Did I mind? Yes, I minded terribly."

The marriage lasted 20 years. Yes, she loved him and, certainly, no one's come near to matching him. But, that said, he needed a lot of looking after and she got fed up of looking after him. She's only seen him once since, at her mother's funeral. No, she doesn't think this odd. "We had no reason to meet again." The children? "Well, they were grown, so there was nothing to discuss there." She never saw Robin again after she called it a day, either. She is good at closing chapters, it would seem.

When she married Robin, an actor she met during some touring job, she was 40 and he was 23. Friends said "Don't do it", but she did anyway. He was very in love with her and, she says, it seemed unkind not to. Sian is not, on the whole, a silly woman. But to marry a man because he'd be upset if you didn't? That's a bit silly, isn't it? "Marriage meant something to Robin, so I did it for him. But you're right, of course, I should have resisted more."

The marriage limped on for 11 years, until Robin went on to America to check out work opportunities and returned with the news he had met someone else. "Robin wanted to discuss the situation. But I said: 'Just get me out of this equation. Now. And if you can be gone by teatime, all the better.' " Golly, I say, that doesn't sound wimpy. No, she says, it doesn't, does it?

She is, she says, blissfully happy now. She has her friends. She has her daughters. She has her cats. She has her canopy. She may or may not have a lover. ("I'm afraid I'm not going to answer that question, because I'll probably lie.") And she has her soaps.

She loves Coronation Street. And EastEnders. And Brookside, all of which she videos and then watches on her telly in the bedroom when she gets back from the theatre. "Yes, it's hard work keeping up, but I love them so." No, she's never been able to get on with Emmerdale. She's not sure why.

She is, she says, never lonely. When she was married to Peter, then she was lonely. When she was married to Robin she was often lonely, too. But living on her own, she isn't. She actually really likes it. She says that when she gets into bed by herself at night, and puts on EastEnders, "my toes curl up with pleasure". She is not, she continues, "some sad old lady who sits at home sifting though her yellowing clippings". She's never kept clippings, as it happens. Although, with Marlene being what it is, she might want to start now.