Talulah Riley: how to marry a billionaire

Talulah Riley has made her new husband a promise. 'I've told him I'll retire with him to Mars,' she says matter-of-factly. 'If he has colonised it by then.' She smiles, but she is coolly earnest. She was once a starlet from Hertfordshire, but she now moves in interplanetary circles. Her husband is the internet billionaire turned space entrepreneur Elon Musk, and her future home is an environmentally controlled pod on Mars. Elon will zoom around outside on rockets, while she makes their biosphere cosy. 'I'd love to get involved with designing habitat systems on Mars – like housekeeping on a grand scale,' she says dreamily.

Today, we are still earthbound, sitting in the Henri Rousseau-like jungle garden of The Beverly Hills Hotel. Flashing on Talulah's ring finger is a diamond as big as Lake Victoria. She shows it to me, beaming. It is the third engagement ring Elon has given her. The first was 'too unwieldy' (more Matterhorn, less Lake Victoria) so, 'bless him', he bought her two others, one for everyday wear, and this one, 'for bling'. 'Elon designed it himself. The diamond in the middle symbolises me and him and the sapphires round it are the ten children.' The ten children? 'I've always wanted ten children.'

Thanks to the wonders of IVF, as practised by Elon and his first wife, Justine Musk, they are already halfway there, with five boys. Justine, a Canadian-born author of sci-fi novels (BloodAngel and Lord of Bones), gave birth to twins Griffin and Xavier, six, and triplets Saxon, Damian and Kai, four. Talulah adores living with them in Bel Air, misses them Monday to Wednesday when they stay with their mother, and would like five girls of her own. 'When I was a little girl I told everyone I was going to marry a very clever scientist and have ten children. I would always draw the children and they included blond-haired twin boys whom I named Theodore and Frederick, Teddy and Freddy for short. It became a family joke, but Griffin and Xavier are those blond-haired twins.' She looks down, defeated for a second by the wonder of it all.

Not so long ago Talulah, now 25, was another aspiring actress fresh from the Sylvia Young Saturday stage school. She was an only child, had a pony, and went to Haberdashers' Aske's girls' day school in Elstree, which encouraged its students to become independent professionals. 'In assembly I remember they read aloud with great scorn a 1950s ladies' advice manual that said, "Put a ribbon in your hair when your husband comes home from work." We were meant to find it shocking but I thought it sounded quite nice.'

Talulah won a part in Joe Wright's Pride & Prejudice, putting in a comic turn as the sweet, tone-deaf Mary Bennet, and then starred in the St Trinian's films, playing new girl Annabelle Fritton. As a single young starlet she lived with her best friend and St Trinian's co-star Tamsin Egerton in a mews house in Marylebone. Tamsin was extroverted and bright blonde, while Talulah was darker and quieter. 'I'm terribly, horribly shy.' I remember meeting her on the red carpet a few times at this point in her life, and she always came across as withdrawn, but poised and perceptive. She says, in fact, that she finds meeting new people very difficult. 'It's hard for me to get used to their physical presence.' She rarely had boyfriends (of which more – or rather, less – later) and was more interested in quantum mechanics. In order to get a grounding in Newtonian physics, she was studying in her spare time at the Open University. Until she met Elon Musk.

Musk, 39, is a South African-born entrepreneur. He was the brains behind PayPal, which he sold to eBay for $1.5 billion in stock. He then founded Tesla, the luxury electric car company, started a solar engineering plant, set up the space exploration company SpaceX and designed the rocket Falcon 9, which NASA now subcontracts. Musk advocates space colonisation as the next evolutionary step for man. He lives in Hollywood and, according to director Jon Favreau, helped inspire the 2008 film Iron Man. When Robert Downey Jr was working out how to portray an engineering genius tycoon, he visited Musk and took notes.

One night in July 2008 Talulah was attending a ball for her favourite charity, the Prince's Trust. Elon was in London giving a lecture to the Royal Aeronautical Society. Somehow they both ended up at the nightclub Whisky Mist (Talulah recounts the concatenation of coincidences that brought them together, involving broken mobile phones and friends of friends, with fond amazement). 'Then there he was, smiling this very big smile and talking about colonising Mars. I was already interested in that kind of thing – the Goldilocks zone of habitable planets and so forth. He showed me all these pictures on his phone: "This is my rocket", "This is my electric car". I thought these were projects he'd worked on, I didn't know they were all his own doing. I did think he was a little bit insane, that first night. In a wonderful way.' She left having promised him dinner.

The next morning, her father called. 'I said, "Daddy, I've met this amazing man who makes rockets." ' Her father, formerly head of the National Crime Squad, now a writer of TV police procedurals, responded with a suspicious 'What's his name?' He Googled 'Elon + rocket + electric car' and exclaimed, angrily, that Elon was married with five children. 'You've been picked up by a playboy. Text that man you're not going for dinner with him and he's a bastard.'

'I was sad, because my judgement on people is usually quite astute,' she says. Talulah rang a friend, who reassured her Musk was already divorced: 'Everyone knows about it.' According to his ex-wife Justine, he had filed for divorce six weeks previously. So Talulah went to dinner, wearing a demure black 1980s businesswoman's dress and court shoes. They both brought friends along as chaperones, and barely looked at one another until late in the evening when they slipped away and sat in Café Boheme, Soho, talking all night. The next day, they had a fit of giggles at the White Cube gallery when Jay Jopling straightfacedly introduced a Chapman brothers' sculpted Pinocchio head with the words 'and this is Bloody Fuckface'. Next she flew out to LA to see Elon, putting herself up at The Peninsula hotel for a week (courtesy of her earnings from Richard Curtis's The Boat That Rocked). After a total of ten days in each other's company, Elon proposed.

Talulah was still a virgin. 'I've never slept with anyone apart from Elon,' she says. 'Which is nice. I mean, which is great. After all that, to describe it as "nice" It's great.' She had had one boyfriend, who was Elon's age, 'who was very lovely to me'. But she always made it clear she did not believe in sex before marriage. Why not? 'For no reason that I can explain. I'm very shy. I don't drink. I had a gulp of alcohol once and it was disgusting – so bitter. I don't drink tea or coffee. I'm like a child, I like fruit juices and sodas and creamy hot chocolate.'

After Elon proposed, Talulah flew straight home to fetch her luggage and her parents. Elon's mother came over from New York and they all had a 'very bemused' getting-to-know-you weekend. Everyone decided a long engagement was indicated. Then the wooing began in earnest. Over the next two years, 'We had a two-week rule, so we couldn't go longer than two weeks without seeing one another. He sent me a lot of sweet notes.
He's very chivalrous and protective; always concerned for my comfort. And he's very romantic. On the first day of filming St Trinian's 2, he sent me 500 red roses on set. And he sent every other woman on set a bouquet so they wouldn't feel left out.'