Indeed, she has part-dedicated her memoir, A Tug On The Thread, to the Love, Actually star who was her partner for 27 years before they announced their separation last August.

They never married, although she used to describe him as her "de facto husband", and neither has ever discussed their relationship. Diana reveals they made this pact long ago.

"It's something we've always been in absolute agreement about - that you have to have a boundary," she states briskly.

They remain the best of friends, she confirms, and the silence which follows suggests we leave it at that.

Today, Diana, 62, looking chic in a crisp, black trouser suit, still retains much of the beauty and elegance she radiated when she shot to fame as the aristocratic Lady Julia Flyte in the 1981 Granada television adaptation of Brideshead Revisited.

Her memoir explores her family history on her father's side, from its beginnings in India and the British Raj, to him coming to England at the age of 18 as an impoverished medical student who became a dentist. Anyone looking for an insider's view of Nighy will be disappointed as, apart from a token snapshot photo, he doesn't feature.

The book is interspersed with anecdotes from Diana's days at Oxford, where she became the first female president of the Oxford University Dramatic Society, her largely theatrical career during which time she was a member of the Royal Shakespeare Company and the National Theatre, and her experiences on Brideshead, the series for which she is best known.

"I nearly turned it down," she recalls. "There was a purity about working in the theatre, almost a snobbery. Theatre was art, TV was commerce and movies were unreachable."

She couldn't have imagined what a hit the 1981 series, which co-starred Jeremy Irons and Anthony Andrews, would be.

"It's about 'nobby' people during the war and Roman Catholicism. We thought it would just pass by. It was well received in Britain but it was its success in America that changed things," she says.

"It meant I couldn't walk down the street for a while without being recognised, but I coped quite well. I was kind of mobbed at times, and it was quite soon after John Lennon had been assassinated so you did feel a little nervous about the enthusiasm."

When it became a hit, it looked like her career was made, but after the first flurry of interest from Hollywood, Diana returned to work in England.

"Rather than sit it out in Hollywood for six months, I came back to work. In hindsight, if I'd sat it out, I think I would have worked on films in Hollywood. I'd love to do movies," she says.

This slight regret is tempered by the fact that she has largely been able to retain her privacy.

"There's a balancing act between becoming a commodity and having a life. I'm quite keen on my privacy."

Her commercial success may have also been affected when, shortly after Brideshead, she fainted on a pavement in San Francisco with her hands in her pockets, knocking out 16 teeth.

"I broke my chin and jaw," Diana recalls. "I came to as my friend was picking me up - and spat out 16 teeth. It was a mess. I couldn't speak properly. It was a year before I could work again."

Her teeth ended up being crowned but she still has problems.

"I spend all of my spare money and a lot of Bill's at the dentist," she laments.

Today, she has homes in London and Suffolk, and informs me that she lives with friends and that she may soon be acquiring a puppy. Her life doesn't seem lonely since her partner's departure.

She may not be a Hollywood headliner, but she's rarely out of work. Yet Diana still harbours insecurities about how long it will all last.

"There are so many good actresses in Britain - Lindsay Duncan, Zoe Wanamaker, Helen Mirren, Penelope Wilton and loads of others - and we are all in the same pool. Once you get past 40 you are suddenly in this very big pool of people. There's more competition for fewer parts."

She'll be starring as the Queen in a forthcoming Channel 4 documentary series which charts five moments of crisis during her reign, with a different cast in each episode.

Looking back, Diana's life has been a series of peaks and troughs.

The third of four children, she had a happy childhood in Kent and went on to Oxford University in the '60s, where life seemed sweet until her father died suddenly of a heart attack when he was only 50. Quick soon realised she knew little about his history or faith when he had a full-blown Roman Catholic requiem mass at his funeral.

"I was completely stunned," she says. "He had seemed so fit and athletic. Then to suddenly find we had this requiem mass at his funeral was a shock. It was as though he'd closed his mind to India and whatever his life had been there."

Shortly after his death she became gravely ill with hepatitis, which left her in a coma for 10 days. Doctors told her mother to prepare for the worst - and she took a long time to recover.

"I was very dizzy and lost the use of my arms and legs for a while, so I did feel disabled by it. I started to feel kind of normal about a year later."

She began acting professionally while at Oxford, but dropped out during her post-graduate degree to go to London to pursue her career - and has never looked back.

"I was incredibly lucky," she smiles. "My first job was with the BBC, which gave me my Equity membership.

"I'm very aware of how much harder it is now. Of course, a face may get spotted and they are turned into a movie star overnight, but it is harder now because there are fewer work opportunities.

"In the late '60s there were loads of repertory theatres and fewer TV companies, but the volume of drama that they did was infinitely greater. There were loads of opportunities to act."

She married - and divorced - the actor Kenneth Cranham and later had a seven-year relationship with Albert Finney, but her longest was with Nighy, whom she met when they were at the National Theatre in 1981.

Their daughter Mary is an actress and director, and Diana is clearly immensely proud of her.

"She started working when she was very young and did her first film in her first summer vacation at university - and this is with her parents saying to her, 'Don't be an actress!'.

"It's very hard, and in these credit crunch times there are going to be less work opportunities."

Today, there seem to be fewer peaks and troughs in Diana's life.

"I've tried to learn from all those ups and downs to be more moderate," she says. "My friends used to joke about me trying to do too many things at once, so now I just do one thing at a time."

She could afford to retire, but cannot envisage it.

"Acting is a vocation - it's great to feel one has a career that is pleasure rather than burden."