"Look at this shirt: it's got a 'no-peep' placket which stops that awful gaping around the cleavage, one of the things women hate when wearing a shirt," says Belinda Earl proudly, taking the crisp white shirt off the rail to show how the layered fabric stretches. Sorry, chaps, but it looks foolproof. The days of flicking your eyes downwards to peep at that bit of the bra may be history. And closing that gap is more vital than ever, as the average bosom has gone up from a 34B to a 36C, and is still growing.
"We've designed it so the fabric stretches when there is extra stress, like when women move their arms around, so there isn't a great big hole showing off their bras. She doesn't like that," says Ms Earl.
The "she" referred to so lovingly also doesn't like jeans that wrinkle around the bottom; cold zips on skin; skirts so short that they show crinkly knees; or sleeveless dresses revealing arms that even the most feminist of feminists will admit can resemble turkey wings.
She is also one of Britain's most powerful shoppers: the Marks & Spencer female. Twenty million people shop in M&S every week, and one in three of them buy their underwear there. So when the M&S Female gets stroppy, M&S listens: and the person with the most acute antennae is style director Ms Earl, who was parachuted in from Jaeger just over two years ago.
The reason that I have gone to M&S's Oxford Street flagship store to meet Ms Earl is that I wrote a grumpy piece agreeing with Dr Ros Altmann, the government tsar for older workers, who recently deplored the lack of decent clothes for women of a certain age on the high street. What's more, I singled out M&S as the worst offender.
Hell hath no fury like a woman scorned – yet, rather than scream, Ms Earl put up a challenge: she would take me shopping to show off the latest collections and prove Dr Altmann and I wrong. It turns out to be the right approach: we've only walked a couple of metres inside the store, and in my head I've already spent a couple of hundred pounds on the gorgeous colour-clashing jumpers and skirts worn by the mannequins standing against the backdrop of its latest ad campaign. The message is subtle but clear: shop here and you are hip – and you'll get help too.
"Customers are always asking for more advice about how to put together outfits," says Ms Earl. "They have a problem with colour and that's a lack of confidence. Mannequins showing off our clothes give them ideas and will build that confidence."
As well as listening to her stroppy customers, 52-year-old Ms Earl – who is dressed elegantly in one of this season's black-and-white slinky dresses – goes high street-walking all over the country to see what her competitors are doing, and to talk to customers: "It's by talking to customers and reading their letters that I learn everything – whether it's that cleavages are a big thing, or cold zips, which we now cover especially. I've been staggered by the passion for the company inside and out, and people take what we do personally. I get that completely."
She's also a stroker, touching every item of clothing as we walk around the store: "I can't help it. I can't stop stroking and touching clothes," she giggles. "When I first came, the sales assistants used to ask me why I was always feeling everything. It's become a bit of an in-store joke now, but it's the only way you can tell if clothes are any good."
Ms Earl agrees that it's not easy being all things to all women – or men. For the M&S Female influences men too: some 70 per cent of M&S menswear is either bought directly by women or with the aid of female persuasion. And now that David Gandy is modelling for the company, sales have shot up.
So how does M&S cater to such a wide audience? "There isn't an average shopper. It's more about attitude," says Ms Earl. "Some ladies dress classic in the day but rock-chic at night. Most are between 35 and 55, but we have something for everyone. There are 70-year-olds who look stunning in the same cashmere jumper as a 30-year-old."
So the store is organising its clothing lines into "mini-brands". There is the Limited edition for the high-fashion younger customer; Indigo for the surfer type; the Classic range for suited types; a Coats shop which has as many colours as Joseph; a Smart Edit section; the Best of British range; and the Moon Harris tweed brand. Then there are the staples: rows upon rows of lambswool and jewelled cashmere jumpers in every pastel shade. Bang go another couple of hundred pounds into my mental shopping basket.
"Our technical team is developing and sourcing new fabrics all the time, and always on the look out for more comfortable fabrics. Even the lingerie lace is no longer scratchy. Look at this new 'sleek and sculpt' vest – it feels like silk yet supports you like a corset without covering the bra."
Pricing is the funniest thing, Ms Earl says: "There is a kudos in saying, when someone admires what you are wearing, 'Oh, I bought it at M&S, and it only cost …' Quality is what the customer cares about most."
So far, Ms Earl has had a magic touch: clothing sales were up last year, and M&S is still the queen of the high street, selling 10 per cent of all clothing and 30 per cent of all ladies' lingerie, despite upstarts such as Zara and Topshop. By the end of our tour, I've mentally filled a couple of trolleys with a leather jacket, or two, a couple of pea coats, all the cashmere jumpers, the stretchy Autograph flowered dress, the entire Rosie lingerie range and much, much more. In fact, there wasn't much not to like, bar the teddy-bear-adorned nightdresses which M&S persist in selling, and which apparently customers love. Apart from the nighties, Ms Earl, I'll eat my words and tip my hat to you: an M&S blue trilby of course.