Testament of Youth: Vera Brittain's daughter Shirley Williams opens up on life and her mother

The book was an instant classic when it was published in 1933 and was heralded as an anthem for the lost generation. It fell out of favour – with its pacifist message – in the buildup to and during the Second World War, and was popularised again with the 1979 television series, starring Cheryl Campbell as the writer. The appeal of the new film – in which Vera is beautifully played by the Swedish actress Alicia Vikander, above – is a reminder that Brittain’s story remains forever fresh.

Williams says that she originally had reservations about another attempt to adapt her mother’s book because Cheryl Campbell was so perfect in the original: “She was wonderful because she caught the spirit of my mother. She had a really inward feeling about it.” But having seen the film, twice now, “I have concluded that it is very well made – subtle, extraordinarily sensitive, brutal when it needed to be candid about what life was like in the First World War hospitals, and not romanticised – I didn’t want it to be and my mother wouldn’t have wanted it to be.”

It was rather amazing to read that when Williams was a young girl, she had aspirations to act herself, and was second choice to play the part Elizabeth Taylor took in National Velvet. I said I loved the idea that it could have been her. She giggles: “You may love it, but it’s a good thing I wasn’t. I would have been very ill made for it, I can tell you.”

Instead, she grew up to read PPE (Philosophy, Politics and Economics) at Oxford (where she became the first woman to chair the Oxford University Labour Club), and was then awarded a Fulbright Scholarship to study at Columbia University in New York. On her return to the UK she became a journalist at the Daily Mirror and then the Financial Times, before entering the political arena and rising to Secretary of State for Education and Paymaster General – holding both positions at the same time, as the only woman in James Callaghan’s 1976 cabinet.

I ask her how she was treated when she entered Parliament in 1964. Was she patronised? “It was more subtle than that,” she says. “It was much more being treated as though you weren’t worth listening to. That was one of the real problems but, to be fair, people like – not just Margaret Thatcher, the usual example – but Barbara Castle even earlier, were quite substantial figures and they stood up for what they believed in and argued the case. They did not fit into the agreeable, supportive woman concept. It’s about being taken seriously and not being treated as a kind of ancillary figure.”

The French can be sexist, she says, “but France is much better at older women. The French treat older women as people with interesting things to say, and are much more willing to accept a woman as intellectually able and with outstanding character.”

People always say when they meet Williams that she hasn’t aged. When you are with her, there is something exhilarating about her engagement and torch-like intelligence – her willingness to beam a light into any issue if there is a chance of it being illuminated, which helps her seem somehow ageless, even at the age of 84. I have never been less worried about asking an interviewee personal questions, for instance, because she has such admirable robustness of character, and will take anything on the chin.

However, she is not a total saint, thank goodness. At one point she makes, what she warns, is a “catty” remark (a very mild one) and she is also not above certain small vanities. For instance, it seems to be important to her that people know both her husbands were famous and great men. In 1955, she married Bernard Williams – described by The Times as “the most brilliant and most important British moral philosopher of his time”. Their daughter, Rebecca, was born in 1961. Three years later she became Labour MP for Hitchin, spending the weeks in London and the weekends in Furneux Pelham.

The rise in her career coincided with her marriage coming to an end in 1974, when her husband fell in love with another woman. For the next decade Williams was a single mother, which can’t have been easy – particularly with the unforgiving hours of politics. Nevertheless, she held three cabinet posts before becoming disillusioned with the Labour Party, and in 1981 was one of the “Gang of Four” who founded the Social Democratic Party (later merging to become the Liberal Democrats) and was the first of them to win a seat, in Crosby, Merseyside.

Her second husband was Richard Neustadt: "He was a very famous figure –a great expert on the US presidency and an adviser to six presidents." She had known Dick and his late wife, Bertha, in the 70s and spent holidays with them in their summer home in Cape Cod.

Shirley Williams

They got to know each other better as colleagues, when Williams moved to the States in the late 80s to become a professor at Harvard’s Kennedy School of Government. In 1987, when she was 57, just before they were due to get married – Shirley’s brother, John, a widower, died of a stroke leaving his teenage children, Larissa and Alexander. Williams and her new husband, who also had children of his own, took them in. What was that like? “Lovely!” she beams. “I’d always wanted a big family and I got one!”

Dick died in 2003, at the age of 84, from complications after a fall. Could she imagine herself falling in love again? “No.” Do you think it would be inappropriate? “No, I don’t think that way,” she says. “I have some very, very close male friends and I have always had a lot of male friends as well as female friends. People who are actually my dear- est friends – who date from 40 years back, and there are people I met at university who I love spending time with – I didn’t have any great sexual adventures with, maybe none. But I found and still find them fascinating people to be with – and their wives don’t take exception to me or think I am a threat of any kind."

"Friendships are very important to me and I think it is quite easy for a woman to make friendship almost impossible because she may never forget the fact that she is an attractive woman, so she is constantly testing to see whether people still think she’s attractive, physically – and that makes it harder to be genuine friends."

“But I wouldn’t say, ‘Give up’ because you never know what amazing thing might happen.” When your first husband was unfaithful to you, was it a terrible shock? “Not really. He was a wonderful man, a brilliant man but he was always a bit...” she pauses, “vulnerable, put it that way. And of course because he was rather good- looking as well as frightfully clever, he was pinioned by quite a number of – I have to say –hopeful ladies.”

And so your marriage ended because of his “vulnerability” – was it really as simple as that?

“It was primarily because of that. I’m a Roman Catholic and I’d been brought up with the view that if you were going to get married, then you had to be faithful – and you only got divorced in the most extreme circumstances.

“One thing I should say, is the first question you must ask yourself if your spouse indicates that they feel that the time has come to break up is, ‘What did I contribute to that?’

“You’ve got to understand how it is that he – who you loved – decided to leave you. And if you don’t begin to understand what you contributed to it, your life will be destroyed by a sense of vengeance, anger, fury, injustice – and, you know, that’s not the way to go.

“You’ve got to come to terms with that single question – what did I contribute to this? And only when you know the answer to that can you actually then seriously think about why you broke up. You mustn’t kid yourself that you’re the one who’s been badly treated – you just have to understand why it happened.”

I wonder whether she has any regrets about not rising higher in politics; she was the frontrunner to be Britain’s first female Prime Minister. “No, I’ve never really had the single-mindedness to become Prime Minister,” she says, decisively. “I’m the Alan Johnson figure of the Lib Dems, I was thought likeable but... He doesn’t go in for political bitchiness and partisanship, and neither do I.”

She is always putting her looks and style down but, actually, I can quite believe that at Oxford, Williams was a shining star with all sorts of substantial men in love with her. As a young woman, she had something of the actress Sylvia Syms about her, and has that fabulous deep, husky voice. It’s not as though she doesn’t make an effort with her appearance, anyway. Today, she is wearing an attractive, heavily embroidered jacket with a Mao collar, which she bought in Srinagar, Kashmir, and a chunky amber necklace.

Please share your beauty tips, Shirley. “My first and most important beauty tip is, for God’s sake, exercise. Absolutely essential. I don’t mean necessarily in a formal gym, but it’s absolutely critical to go walking and swimming. I still walk several miles at the weekends and I still swim a couple of times a week.

“The other one – which will go down very badly with the beauty companies – is that I always wash my face with soap and water and I don’t put anything else on.”

Her secret to a happy, healthy, fulfilled old age is so instructive it should be made available to all, as part of Baroness (“Just call me Shirley”) Williams’s Lessons for Life: “I think a lot more about the future than the past and I find that most people of my age group think very largely about the past.

“It is tempting because it means that you don’t have to think about problems that confront the world and you can safely roll back to when you were young. But I find what is going on now fascinating. My family, including my grandson, talk about politics with me so I’m rather conscious of what’s going on with the younger generation.”

It’s time for her to go and make waves in the House of Lords, where she has led a revolt (something she has done on a number of issues) against the attempt by the Government to alter the right to judicial review. Afterwards, she has to go to a function to meet someone high up at the BBC, and something else after that – before she even thinks about bedtime. What a woman.

Testament of Youth is on UK cinemas from today (Friday 16th January)