Who could not be obsessed by a woman as exotic and captivating as Lady Idina Gordon? As I read The Bolter, the new book about her life of wanton excess, I knew immediately why Frances Osborne had chosen to write about her. When we met for lunch Frances told me that finding out about her glamorous great-grandmother had changed her life: “I was a clever, swotty girl, and Idina gave me the confidence to dare to be something more.”

This rebellious streak has lasted. Frances claims that despite being a mother of two she is still naughty and giggles happily about how she used to zoom between west London and her job in the City leatherclad on a motorbike, and about being a keen smoker between lessons at Marlborough college. “Isn’t that what those five minute breaks were for?”.

Perhaps because of the shadow of the Bolter legacy (her mother was haunted by it), Frances can seem reserved, but underneath is a great sense of mischief, bursting to escape the caution she must necessarily exercise as the wife of a coming political force. Her husband is George Osborne, right-hand man of the Conservative leader David Cameron and the man set to be chancellor of the exchequer if the Tories win the next election.

In jeans and an orange tank top over a crisp white shirt, she looks carelessly yummy mummy and, like her friend Samantha Cameron, she takes a no-nonsense approach to the duties of a modern political spouse.

What, I ask, are the dos and don’ts of the role?

“I’m the worst person to ask on that, I just don’t really do it,” she says cheerily. She must have done her time on the rubber-chicken circuit to help George get elected, surely?

“Pass,” she giggles, “I don’t really go. I see all that as being George’s job, and he’s good at it and he’s elected and I support him as I would as if he did any other job. I will do a few things, as most wives would, but now we’ve got the children I’m tired and I don’t think they need both of us on parade. It’s his job not mine.”

So there’s no Clinton-style, two-for-the-price-of-one here. Perhaps her own background (her father was a Tory minister) fostered her down-to-earth perspective on politics.

“Yes, it’s very familiar to me. George is very efficient; the other day he’d written a piece before breakfast. My father was like that too, he’d whizz off and write speeches and things.I don’t think I could have fallen in love with someone who was only interested in making money for themselves. I could only fall in love with someone who had wider horizons, who did something for the good of the wider world.”

Will her husband be any good at handling the national finances? What’s he like with the Osborne household budget? “Are you talking about paying the bills and things? I do all of them. Well, he pays some of the Cheshire bills [the house in his constituency].”

I presume he chooses the mortgage and that kind of bigger thing?

“Oh no. I do all that.”

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