It’s a cold and blustery morning at Mumsnet headquarters in Kentish Town, North London. For an organisation that boasts more than 800,000 users and apparently has the power (almost) to bring down prime ministers, it’s a remarkably restrained set-up. No liveried doormen here, just a single security man and a rented space in a shared office building.

Behind the door of Unit 6 sit a dozen or so ordinary-looking women, beavering away on their computers. The atmosphere is calm, tidy and a little subdued. The average age is mid-to-late thirties, the clothes relaxed but elegant, hair well cut but not ostentatious. It does not feel like a hotbed of political unrest. Indeed, a less intimidating group of women you could not hope to find.

Justine Roberts, 42, a mother of four, is the co-founder of Mumsnet. Whippet-thin and casually dressed in jeans and a shirt, she is busy setting up a conversation thread, inviting thoughts about the Queen’s Speech from her “mumsnetters”. Given the site’s reputation, I have come prepared for extreme bolshiness. I find none, just smiles all round and the offer of a cup of tea (no biscuit).

Where did Mumsnet come from? “It was when the twins were about a year old. We went on one of those so-called family-friendly holidays and the childcare was really bad. All the parents were sitting around complaining, and the idea just came to me: wouldn’t it be great to have a site where mothers could swap tips about this kind of thing.”

At the time she was working as a freelance sports journalist but her background was in the City, as an economist and strategist. “Having worked on Wall Street I knew I could never go back to the City,” she says. “So I dragged in Carrie [Carrie Longton, her co-founder], an ex-TV producer who was on the same antenatal course as me.”

Mumsnet went live in March 2000. Within a year it had 10,000 users. Now it has nearer 850,000. It doesn’t make much money but that doesn’t seem to be an issue. Anonymity is a core principle. It allows women to talk freely.

So how did a site designed primarily for the exchange of baby-related information grow to become a target audience for top political strategists? What constituency does Ms Roberts represent? “Our women are on the whole quite educated,” she explains. “Not necessarily wealthy, but 75 per cent of them have a degree and two thirds of them work.” Middle-class? “Put it this way: our most successful affiliate partner is Boden.”

Even the most cursory visit to the site reveals a group of highly opinionated, socially liberal and environmentally conscientious people. As any frazzled mother who has used the site knows, they can be kind, thoughtful and supportive. They can also be bossy, smug and even a little scary. The tone for yesterday’s webchat with Mr Cameron, for example, was openly hostile. “Mumsnet is not a soft touch,” says Ms Roberts. “It is full of women who by definition are well informed and confrontational. How politicians do on Mumsnet is not just about getting the answers right, though. It’s about whether they’ve been human, whether they get the joke. The difficulty for political parties is how to interact with the whole idea of social media in a non-embarrassing way.”

However individual politicians fare on the site, there is no doubt that for strategists interested in accessing the minds of a certain type of female voter, the site offers an idea of the female psyche at its most raw (and occasionally emotional) form. It is perhaps telling that traffic peaks between 9pm and 10pm: what Ms Roberts calls the “Mumsnet and wine” hour.

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