Are you as posh as the parts you often play?
Not at all - I went to a comprehensive, left school before I was 16 because of undiagnosed dyslexia and earned a living by cleaning houses before I applied to drama school. It frustrates me that I get locked into that posh image sometimes.
Did you find any redeeming feature in your latest TV character, Anthony Trollope’s scheming Countess De Courcy?
I have to admit I rather enjoyed playing someone with enough self-confidence to believe that she is always right! It’s a satire about people’s obsession with status.
Related: Read our TV critic's review of Doctor Thorne
Is there anything you don’t like about doing costume drama?
Wearing corsets. I’m always shocked at how much those brutal contraptions imprisoned women - and often shortened their lives because they had no air in their lungs.
What do you usually wear around the house?
Jeans and jerseys; I like clothes but they’re not that important to me.
What did turning 50 mean to you?
As an actress, it meant I was suddenly playing mothers. But I’m not worried about getting old, so long as I’m always fully awake to life by listening and learning.
The best thing about becoming a grandmother in your 50s?
I have enough energy for my two grandchildren. It’s an extraordinary relationship and I embrace it and love it; they are very, very special.
What was your big break?
I had worked as a child actress from the age of four, but my real breakthrough came when I met the director Michael Lindsay-Hogg - who cast me in a West End play at 20 and then in Brideshead Revisited at 22. When you are young, you need someone to believe in you and champion you, which he did.
Did you have a Plan B?
I always felt I couldn’t do anything else except act… my parents were both actors.
My family: my husband Charles Sturridge, my children [the actor] Tom, Matilda and Arthur and my grandchildren Marlowe and Rudy.
When did you first realise you were falling in love with Charles, who took over from your mentor Michael Lindsay-Hogg as director of Brideshead?
I had a crush on Charles very early on. He was unbelievably good-looking, articulate and clever - and filming Brideshead with him was the happiest, most euphoric and hedonistic time of my life.
It’s very easy when you are young to fall in love with your director, since they are people who seem in control; but since the filming lasted for two years, on and off, because of a technicians’ strike, we got to know each other very well - and eventually married in 1985.
Which decade makes you feel nostalgic?
None: I always feel you have to live in the present, so I’m never nostalgic.
Wires! Our house is full of chargers and adaptors for all the computers and smartphones, so there are lots of wires everywhere that look like mating snakes and drive me insane. I’m always losing my chargers.
Books or e-readers?
Books - I don’t have a Kindle.
What music gets you on the dancefloor?
My children are always telling me not to [dance]! Although I had to do a 19th-century one in my latest drama, Doctor Thorne. But for dancing in real life, it would have to be The Beatles - or Leonard Cohen for a slow one.
Paul McCartney and George Harrison, the best-looking Beatles. I went to two of their concerts with my 60-year old aunt.
Four girlfriends whom I’ve known for decades; they are hugely important to me. Some are actors, some not.
My husband and I are on our seventh house move in 30 years and I’m throwing everything out at the moment; decluttering is the best detox you can give yourself. But a clay head that was made by a friend who died is still incredibly precious to me.
Is there anything else you can’t throw away?
My children’s childhood paintings - and all the letters my late mother wrote from the age of 16 to the end of her life. She didn’t throw anything away, which drove me crazy when she was alive, but I’m so grateful to have found all her letters since then; it was so touching to read them.
That’s what’s sad about email: it’s so ephemeral - and not very romantic. There’s something about opening an old envelope and reading a person’s character from the handwriting, which gives you such a clear picture of their personality.
Not being as adventurous in life as I should have been; I haven’t travelled enough, though I’m trying to make up for it.
How do you relax?
There’s nothing better than an early night with a book in bed.
The John Williams novels Stoner and Butcher’s Crossing, the short stories of Richard Yates and A Fine Balance by Rohinton Mistry, the Dickens of India. And Trollope - an incredibly modern writer for his time.
What advice did your parents give you?
My father advised me always to look ahead and focus on what’s in front, not worry about anything else.
What makes you grumpy?
The list would go on far too long if I told you! But I try not to be too negative.
Exercise, good diet or both?
I do a lot of walking, which I love, and I eat healthily. I go to the gym very occasionally but the habit never lasts long.
I’m a good cook.
Ideal dinner party guests?
That’s a no-brainer…my family.
Verdict: Anyone who has enough energy to move house seven times and dance to The Beatles has to be a youthful 48.
Julian Fellowes’ three-part adaptation of Anthony Trollope’s Doctor Thorne is on ITV in March.
A version of this article was first published in the March 2016 issue of Saga Magazine. For great articles like this, subscribe to the print edition or download the digital edition today.