Keeley Hawes, the star of Spooks, Tipping the Velvet and, in posh voiceover only, the Lara Croft computer games, is full of surprises. When we meet for breakfast, she starts telling me how she was up at 4am to feed her son. As I have read about only the one son, Myles, I conjure up a Little Britain-style picture of the six-year-old guzzling greedily at her breast.

Tall and, so far as I can tell beneath her grey top, slim, with clear green eyes, impeccable skin and blonde hair darkening fashionably at the roots, Hawes certainly doesn’t look like a harassed new mum. Yet, it transpires, in September she gave birth to a whopping new baby by the name of Ralph.

“He is 10lb 8oz, nearly as big as you,” she says giggling with mock horror. “He came out and everyone went ‘Oooh!’ and I thought three heads or something, but he was just enormous. He doesn’t look like either of us. He looks like Jeff from Curb Your Enthusiasm, you know, the fat agent.”

It was, despite everything, a reasonably easy delivery and — since most of her neat bump had contained simply Ralph — within a few days her tummy was back to trim. But it is not so much her weight as the absence of baby blues that makes Ralph’s arrival hard to credit. She laughs so much in our hour together at Hazlitt’s Hotel in Soho that I think she must have post-natal euphoria.

Ralph is her third child, the second by her second husband Matthew Macfadyen who, like her, became well-known through playing a spy on the BBC’s Spooks. Last year he leapt into true fame as Mr Darcy, playing opposite Keira Knightley, in Pride and Prejudice. She is clearly very much in love with him. After I leave, she apparently talks non-stop about Matthew to our photographer. But, then, after some dark times — which we can hardly avoid discussing — she seems in love with her whole life at the moment.

There is, however, a pleasing irreverence, even a cynicism to what might otherwise sound like gush. Although on Boxing Day she is in After Thomas, an affecting ITV tear-jerker about an autistic child saved by his relationship with a pet dog, she doesn’t get at all misty-eyed talking about it. She plays the mother and began filming when eight weeks pregnant. Did it not fill her with worries about the kind of child she might have?

“A child actor?” she replies, quick as you like. Relations between her and six-year-old Andrew Byrne, who played the autistic boy, appear to have been a little bumpy. “He’s in most of the scenes and he had a funny little double as well. There was him, a double who was lovely and wore a wig and — I don’t know really whether I should say — a girl, a beautiful little girl, a midget, as a stand-in. So there’d be three of them all dressed the same, and a dog, and some puppies, and a dog-wrangler and a child-wrangler. So now I know why people say don’t work with children or animals.”

The family on which After Thomas is based visited the set. I ask what the real mother thought of her portrayal. “She actually arrived at the moment [Andrew] punched me in the face. He wasn’t supposed to. It was just very naughty and it was the one moment when I broke because we’d rehearsed and rehearsed and just in the middle of it he punched me in the face. It was a scene with the dog and I was wrangling them both and I did snap and it was very naughty of me.”

Did he cry? “He didn’t cry. He was quite surprised, though. I don’t think he’s used to people raising their voices to him. Anyway that was the time when they decided to arrive. So they were watching the scene on the monitor and I went over and said, ‘Hello, nice to meet you’, and she was going (Hawes makes a thumbs-up gesture.) I think that as a parent of a normal child or an autistic child you’re patient, you’re patient and occasionally it wears thin.”

Hawes recounts all this in her tinkling cut-crystal voice that makes her a favourite for voiceovers. Another surprising thing about her is that it is not how she always spoke, but the result of elocution classes taken at the Sylvia Young Stage School.

She was brought up almost opposite the school in Marylebone, Central London, and begged to be allowed to audition for it. She was there from 9 to 16. Her father, Tony, is a London cabbie who, she confirms, talks like a London cabbie. Her near contemporaries at the school — the Spice Girl Emma Bunton, the Appleton sisters and Denise Van Outen — clearly took the lessons less seriously. But nothing about Keeley Hawes is a s surprising as her personal life. An early profile in Elle, at the time of her (mis)casting as Diana Dors in an ITV biopic, avowed: “No tabloid exposés (à la Diana Dors) or celebrity flings are likely to taint Miss Hawes’s reputation.”

At the time, 1999, she was with the graphic artist and cartoonist Spencer McCallum, whom she married not long after Myles was born. Shortly afterwards she went off to film the first season of Spooks for the BBC. There she met Macfadyen, spy Tom to her spy Zoë. One day, filming in the rain, he declared that he loved her.

She had been with McCallum for seven years and married to him for eight weeks. She has always insisted that she did not succumb there and then, but within months she had moved out of the family home in Esher, Surrey, and moved to St John’s Wood, North London. Macfadyen remained chastely in his flat in Shoreditch.

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