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But this scheme required £300 and the BBC bean-counters turned her down. So Baxter marched past the secretaries and “no admittance” sign straight into the office of Donald Baverstock (then assistant controller of programmes), and demanded to speak to him. “It sums up Biddy,” says Edward Barnes. “Ruthless and totally fearless.” He chuckles and adds, “Baverstock must have been bowled over by this tall, striking blonde bird – 5ft 8in, always wearing these three-inch stiletto heels – and Biddy got her badge.”

Perhaps even Baxter herself didn’t foresee the full impact of her idea. The galleon crest became the logo of TV’s first mega-brand. Moreover, to acquire the precious badge, children began to deluge Blue Peter with letters, providing a perpetual focus group on the programme’s content and a wellspring of ideas. “If ever you were short of an item,” recalls Rosemary Gill, “you just had to rootle through the post. And there might be a letter from a child who’d trained her dog to jump through a hoop. And from there you had a feature on dog training or pets who could do tricks…”

The letters collected in Dear Blue Peter form a social document which shows that although the world has changed, pre-pubescent children, their passions, tastes and creative urges, have not. And it was the synchrony between programme and audience, the children’s sense of being heard, included and valued, which built the audience up to a peak – in those long-gone four-channel days – of eight million.

For Baxter, Blue Peter was all-consuming. “She was a 100-percenter,” says Edward Barnes. “And she expected everyone to be the same.” In the office by 8am, she read all the national papers and many local ones. (She was partial to following up stories in the Leicester Mercury, not just because it was her hometown paper, but, fearing accusations of London bias, she saw it as a middle England touchstone.)

“We had to be one jump ahead,” she says. “Children don’t understand the finesse of doing a subject in a different way. If you had Uri Geller on Tomorrow’s World, even if he did something totally different on Blue Peter, they’d think, ‘I’ve seen that.’ So we had to have everything first, and if it was something like the first UK appearance of the Chinese Acrobats, we’d do a deal and get them.”

She sounds, I say, like a newspaper editor. “Yes, it was absolutely analogous. The only thing that we didn’t touch was politics, because children found it so unbelievably boring.” Hard-hitting subjects – famine in Biafra, the Holocaust, post-Pol Pot Cambodia – could be packaged for a young audience. She talks of how children enjoy “stretching up” to grasp a difficult or unfamiliar topic, how much better that is than underestimating their intelligence.

If you want to inflame her, suggest Blue Peter was middle class: “It is a load of utter codswallop. They are calling information middle class! We had viewers from every income and part of the world.”

Athough Blue Peter is assumed to be prim and nannyish, she had a surprisingly blasé attitude towards sex. I find myself devising Daily Mail headlines for items as far back as the late Sixties: children being asked to choose from a line-up of dogs which one should mate with Petra, Tina Heath having her pregnancy scan on TV, a truly weird Blue Peter annual cover in which Noakes and Purves are topless in circus costume, the former wearing little but a very tight thong.

Baxter rarely left the office before 7pm and always with a huge basket full of files. I ask if she had time for a life and, slightly irritably, she informs me, “I enjoyed music and opera. I had lots of friends. And I quite often had weekends without the telephone going.”

She expected her team to share her dedication, particularly the presenters, who performed live with no autocue, Baxter often switching the running order minutes before transmission. Val Singleton, Peter Purves and John Noakes have all complained in recent years about being underpaid, overworked and never allowed to contribute to the show’s content. I detect that Biddy is hurt by this, particularly by Val, whom she still has lunch with. Singleton lately revealed she had a one-night stand with Purves, a fact which makes my inner child recoil into a foetal ball. “I was not aware of that,” Biddy says tersely.

But if Blue Peter was so irksome, why did they renew their contracts every year, she asks. Perhaps, she demurs, the first presenters stayed too long, burned out, damaged their long-term careers. More recent presenters – who tend to stay for only a few years – are generally kinder about their time on the show. It suggests Biddy grew to understand the pressures of instant fame and remaining “squeaky clean” (“I hate that expression”) to be a role model for young fans.

Sarah Green says that Baxter was a lioness, protecting anyone who shared her passion for Blue Peter, but unforgiving of those who betrayed its audience. Simon Groom once concluded an item on hedge-laying with the link, “Yes, Sarah, if you want a good lay, you need a good length,” and scarpered before heels clacketty-clacking from the gallery, furious at this double entendre, could catch him.

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