The mark of a truly great shop is the way in which the staff know, almost telepathically, what you want, even before you do. And I’m not talking about those algorithms used by iTunes that seem to think my next purchase should be either Frank Ifield – The Essential Collection, or The Crazy World of Stan Freberg. (It also came up with Crosby Still and Nash’s Deja Vu, but I already have that).

No, I refer to a men’s clothing retailer that not only understands my taste, but factors in variables like the weather, and my fluctuating waistline. This intuitive skill is what makes Marylebone’s J.Simons such a rare gem. That and a stock of the most wearable garments any man, of any age, could ever hope to own.

On a visit some months ago John Simons, the quietly spoken founder and driving force behind the company, that opened its first shop in 1964, greeted me warmly, perhaps in recognition of my being a customer for going on 25 years. After the initial chit chat he said he had something that would be perfect for me. Moments later he produced a vintage bespoke suit, in black barathea, that, frankly, fits me like a glove; not just the shoulders and sleeves, but the waist and even the trouser length. Who needs digital mapping with eyes as sharp as John Simons’?

If I were to describe J.Simons as an American outfitter, which is largely true, you would probably have the wrong impression, unless you already acknowledge that Americans have a knack of taking quintessentially English looks and handing them back to us, sharper, often better made (over engineered like their motorcycles), and indisputably cooler. The genre is ‘Preppy’, or ‘Ivy League’ – derived from the clothes worn at prestigious East Coast universities and preparatory schools. A look that has never, and will never, go out of fashion, for the simple reason that it is so timeless.

Check shirts and chunky knit sweaters in the shades of a New England fall, and shawl collar cardigans. Brooks Brothers polo shirts, and Baracuta (aka Harrington) jackets, Grenfell raincoats, manly boots by Red Wing and Sebago, and triple welted brogues from Florsheim. And J.Simon’s signature garment, the button Oxford cotton shirt, the backbone of any modern man’s wardrobe. “The roll of the collar, that’s the most important thing,” reads a hand written tag in the window.

For men who cite Cary Grant and Steve McQueen as style icons and marvel at the perfection of Miles Davis’ green button down shirt on the cover of the 1958 album ‘Milestones’, J.Simons is the Holy Grail. Here is the missing link between Savile Row and Hollywood. Mid-Atlantic tailoring for men who want to be smart – but never stuffy.

But J.Simons is much more than a place with the right brands. The store’s unique talent is knowing its customers. Simons tells me about one, who moved to the US some years ago, and who annually and asks him to source, by phone, some trousers, shirts and maybe a jacket or two.

“He trusts me,” says Simons, picking out items he deems suitable for the California climate.

I trust him too. Each time I step into his new shop in Chiltern Street, after a generation in Covent Garden, he’ll look me over, allow for a modest shift in size, and then present me with the perfect tweed jacket, a pair of Bermuda weight dog-tooth slacks, or some Venetian ‘dress’ loafers. And, be bang on the money in size, shape, colour, and ‘look’ – every time.

“I match people to clothes,” says Simons, professorially. “It’s a knack.”

J.Simons,  46 Chiltern Street, London, W1U 7QR. 020 3490 2729/johnsimons.co.uk