Sian Phillips brings tea and chocolate biscuits. Both are excellent, but she gets flustered because the tea is weak — “I seem to have lost the knack” — and apologises that the biscuits are M&S. But this is fine by me and, in fact, no problem for her, as she’s on the cabbage-soup diet and isn’t allowed biscuits by M&S or anybody else. “The first day is just steamed vegetables and salad, the second day is all fruit and the third day is fruit, steamed vegetables and salad.”

“And, er, the soup?” “Oh, you have that three times a day anyway.”

Doubtless it works, but it doesn’t seem strictly necessary. Phillips looks very slender. But how slender is slender enough for Miss Havisham? She is playing the greatest mad old bat in English literature, in the Royal Shakespeare Company’s new production of Great Expectations. And Miss Havisham is plainly supposed to be wasting away, so, just to be sure, it’s cabbage soup.

This is a big moment for Phillips, for the RSC and, in a strange, roundabout way, for the whole of the British acting profession. The last time Phillips appeared with the company was in 1959, in the midst of the worst year of her life. She’d just married Peter O’Toole and given birth to the first of their two daughters.

“I have a feeling I had postnatal depression. It was rarely spoken of in those days. I lost confidence in my work. I was lucky to have people in the company who were good to me. But I had to go to London, and I had to leave the RSC, which was a terrible shame, as my natural home as a performer would be in a company... I’m very institutionalised. I think I’d probably be quite happy in a prison.”

Exiled in commercial theatre in London for the next couple of decades, she lived a tempestuous and well-documented — not least in her autobiography, Public Places — life with O’Toole before they finally divorced. She got a CBE for her efforts, but didn’t become one of our grand theatrical dames, probably, it is said, because she was denied the chance to build her reputation in the RSC.

“Oh, I don’t think so...”

Now she lives in a modern Islington flat with a gigantic sloping window that, as we speak, darkens with the onset of dusk. I feel in the crepuscular gloom that we are conducting a seance, which, in a sense, we are. There are many spirits about.

Born in 1934 in deepest Carmarthenshire, she spoke Welsh and French before landing elegantly on English, a progress that perhaps explains the alien perfection of her diction. Neither of her parents were quite what they should have been. Her father, a steelworker and policeman, should have been a singer, and her mother should have been a teacher, but had to give up when they married. They had underused gifts, but weren’t in any way showy. “They were very puritanical — Welsh, extremely Welsh. They were the least showy people imaginable.”

Phillips was brought up as an only child; a sister died before she was born. It was that experience of her mother’s, combined with her decision to have another child, that has, latterly, filled Phillips with a kind of wonder at the character of a woman she plainly feels she did not know well enough. “I think about my mother all the time now, much more than I did when she was alive, I’m afraid. We had a sort of tussle, we never had a row, oh my God, no. Probably she was a great influence on me, and I really did admire her.”

The atmosphere at home, though stern, was not especially religious, and by the age of 13, Phillips no longer attended chapel. She was bright and won a scholarship to the University of Wales to study English and philosophy, though by then she knew she wanted to be an actress. It was her mother who made her take the degree. Phillips had been on the stage, and she had even been on BBC television, acting and reading the news. She married, but only briefly, because she got into Rada and left Wales, marriage and home. “Getting married was just going with the flow, really. I wasn’t thinking about anything at all; I was frustrated and couldn’t think how to get out.”

And so she starred at Rada, married O’Toole — that’s what she calls him, never Peter — and joined the RSC, to run into the brick wall of the worst year of her life. The ensuing 20 years, in which she did a lot of commercial theatre and television, and ran a film company with O’Toole, were what they ultimately became, excellent memoir material. Phillips brought up their daughters in a house in Hampstead, went to the West End in the evening, endured O’Toole’s binges and was periodically called up and asked to take visiting Hollywood stars to Tramp or Annabel’s.

“Everything went very right in many ways, a lot of things were very good, but my working life went wrong, yes ... I hated Tramp and Annabel’s, so I just got them in and went home. It was really silly. I hardly even knew them.