“But the wedding night didn’t exactly go as planned. We were in bed when we
heard this moaning from the corridor: it was our drunken best man, who ended
up getting into bed with us for an hour and a half. There I was thinking:
‘I’m pretty sure this is not how things are supposed to happen.’ ”

Ecclestone met 26-year-old Stunt on a blind date five years ago and the pair
have tried “never to spend a night apart” since.

How does she know if someone’s after her for her money? “You can always tell,”
she says, hard all of a sudden. “It’s obvious if a guy wants to use you to
become famous, and James doesn’t. And he can support himself, which makes
life easier. If we go on holiday, he pays, so I don’t have to lug him around
and pay for him. But those weren’t the only reasons I was attracted to him.”

Watching her 27-year-old sister, Tamara – more outspoken and publicity-hungry
– suffer the indignity of a “kiss ’n’ tell” taught Ecclestone to be wary.
“I learnt through her that people can turn evil, bitter and jealous once you
break up. But I’m lucky because I never really had a serious relationship
before James.”

For all her pampered upbringing, Ecclestone was forced to grow up fast. She
attended her first couture show (Dior) at 10; “came out” at the Crillon Ball
in Paris at 16; and spent a lot of time at her parents’ dinner parties as a
child, “behaving like an adult”. You sense that with her controlled
demeanour and precise enunciation she’s still doing that now; that beneath
the glossy façade – the false lashes, Balenciaga trousers and Helmut Lang
blouse – is a Chelsea girl more at home watching telly in Ugg boots than
swilling Krug at Tramp.

In 2009, after 24 years of marriage, Petra’s parents divorced – “one of the
most difficult periods of my life”, she says. To keep her daughters
grounded, 6ft 2in Slavica eschewed the help of a nanny or dishwasher, and
regularly took her girls back to the Croatian port of Rijeka to meet their
relatives. “My grandmother worked on a fruit and vegetable market stall,”
says Petra. “Showing us where we came from was important to Mum.”

Aside from a brief period at Francis Holland girls’ school – where she wore
tracksuits, put on a mockney accent and changed her name to “T” – Ecclestone
never rebelled. She doesn’t take drugs and doesn’t drink (“the only thing I
like is Malibu and pineapple because it doesn’t taste like alcohol”). Now,
after a spell as a teenage model and creating a menswear line, Form, which
was victim to the recession in 2009 – she is keen to become a businesswoman.

I’ve been told not to mention “the house” – Aaron Spelling’s former home – but
all this Zen-living baffles me. What, exactly, will a 22-year-old and her
new husband be doing in the mansion’s estimated 123 rooms and 27 bathrooms?
Is she planning to start a (very large) family?

“I definitely want three or four children before I’m 30,” she nods, breaking
into a grin. “I think I keep buying dogs to replace children. But I saw a
lot of houses and so many were like museums. This one is quite cosy – it’s a
real family house.” And a good investment, she insists, despite barbed
comments from her sister’s boyfriend, Omar Kyhami, on how the house, which
had been on the market for two years, was a financial “disaster”.

“We got a good deal. I mean, how many people over the past few years would be
able to afford that house? Not many – that’s why it had been on the market
for two years.”

After two months of intensive renovations, “the Manor” is unrecognisable, she
says. “We’ve painted it all dark colours – even our bedroom is black.” Would
she mind a quick game of “truth and myth”? “No,” she laughs. “I’ll start: I
didn’t ‘buy the house for my dogs’.” Good to know. And the whole floor of
wardrobes? “Doesn’t exist. There’s a big room – not a whole floor.”

Designer clothes, she admits, are a weakness, “but it’s shoes that I’ve built
up quite a collection of”, she grimaces. “Every three to four months I ship
a whole load over to Croatia for charity.” So in one small Croatian village,
all the women are wearing Louboutins? “Yes,” she giggles. The built-in spa
one newspaper included in the mansion’s list of excesses does exist, she
confirms, but there will be no £1 million crystal bath tub like the one
Tamara recently said she’d bought for her London pad. “I don’t even know if
that’s true,” Ecclestone says, shaking her head.

Although the sisters claim to be close, the elder Ecclestone sparked rumours
of sibling rivalry earlier this month when she arrived in LA and announced
that she would be house-hunting, “starting at the £100 million mark”. “Am I
at war with my sister?” said Tamara. “You’ll have to ask Petra.”

“People always want to think that there are problems,” Ecclestone says. “But I
don’t see any sort of rivalry. Tamara loves…” she pauses, “not the limelight
but having her picture taken and performing, whereas I’m shyer.” And yet the
younger sister knew nothing of Tamara’s supposed plans to move to LA until
she read it in the papers. “She’s never told me that she’s moving here,” she
shrugs, “so I don’t know what’s going on.”

Although Ecclestone was unable to avoid appearing on her sister’s forthcoming
TV show Tamara Ecclestone: Billion Dollar Girl (“they were filming the whole
time we were at the Monaco Grand Prix together”), she’s anxious to make a
distinction between their mutual ambitions.

“I prefer to be behind the camera, designing,” she tells me, citing Jimmy Choo
co‑founder Tamara Mellon as her inspiration. By the time she’s 30, Petra
says she would like Stark – her collection of 35 edgy handbags, with prices
ranging from £495 to £2,500 – to have burgeoned into a full-time career.

“I don’t want to aim too high, but hopefully I’ll have handbag and shoe stores
around the world.” The worry, she laments, is that whatever success she does
achieve will be attributed to her parents’ funding. “Of course money makes
things easier, but it’s not going to make anyone want to buy my handbags.”
She looks down at her untouched salad, suddenly deflated. “I don’t mind what
they call me. They can call me a prostitute as long as they don’t say that
my full-time job is spending my father’s money.”

I wonder whether it’s not the public perception but a fear of living up to her
father – the 80-year-old billionaire who started out selling fountain pens
in Petticoat Lane – that plagues her most.

“I’m very like him,” she says slowly. “I don’t like the idea of waking up and
not having anything to do. I don’t need to work, but I’ve chosen to. Do
people think I don’t know that I’m privileged and lucky for what my parents
have achieved? I’m just trying to be successful, and even though I know I’ll
never be as successful as my dad, I’d like to make my parents proud.”

The Stark collection will be available from selected UK retailers from
January 2012