If you went on a diet and exercise regimen that made you healthier than you’d ever been in your life, you’d be proud to show off the results. You’d pull in your belt a few notches, stick your chest out, turn your honed features to the light. Yet Mercedes has just got the E-class into the shape of its life and, for some reason, appears not to want anyone to know about it.

I guess if you already owned one or happened to have a particular thing about this model, you might notice that the grille, bumpers and headlamps are slightly different, but otherwise you’d have no clue that underneath this middle-aged saloon lies a car transformed.

Mercedes says that more than 2,000 components have been replaced or renewed in the midlife upgrade — a farcical assertion, I thought, until I spent a few days behind the wheel of an E 500.

It still has a V8 motor, but though the old 5 litre one with 302bhp was not exactly lacking in performance, the new one, with 5.5 litres and 383bhp, reduces the 0-62mph time from 6sec to little more than 5sec.

Almost unbelievably, that makes this staid-looking sedan only a sliver slower than a Porsche 911. With a standard seven-speed automatic transmission throwing gear after gear at the rev-hungry engine, it’s terrifyingly easy to turn what started as a harmless squirt away from a roundabout into something seriously prejudicial to your driving licence.

And this isn’t even the fastest one: the old flagship E 55 AMG is now the E 63 AMG with 507bhp instead of a trifling 469bhp.

More important than the power boost, the chassis has been comprehensively updated too. Gone is the old vagueness in the steering, replaced by a delightfully direct and accurate system. There’s less roll in the corners, less wallowing and heaving over dips and crests, yet its ride comfort, the thing you’ve always been able to count on an E-class doing better than any of its rivals, has suffered not at all.

Yet for all this you’d never call it a sports car in saloon body. Its great rival, the BMW 5-series, is still more fun to drive, more responsive and receptive to the needs and wishes of the true enthusiast driver, even if it makes you pay for the privilege with an uncompromising ride quality.

For all its new-found strengths, however, the sense of déjà vu that pervades the new E-class experience is hard to escape. The car is four years old now and looks every day of it from the outside — particularly with opposition such as the Audi A6 and Lexus GS 300 looking so modern and smart.

The same can be said of the interior. The cabin is assembled from high-quality materials but it looks old: the centre console is a smother of switches, making the navigation and information screen with its antediluvian graphics needlessly hard to understand.

It has two other problems. Those who had traditionally bought E-class saloons but who don’t need space for five adults can now choose the gorgeous CLS coupé, which not only has four doors and enough room in the back for most family purposes, it also looks as avant-garde as the E-class looks ancient. Based on the same running gear, it should be at least as good to drive. Those who bought the E-class estate may meanwhile be wondering if the new six-seat R-class luxury MPV would not suit them even better — when it comes to carrying either big loads or lots of people, it is undoubtedly superior.

Me? I’d still have this E 500 because I actually like the fact that there is almost no discernible visual difference between it and any car on any taxi rank outside any airport in Germany. It may look old, but it is also discreet and lacking entirely in ostentation.

I’d remove the badge from the back, waft around in silence and, whenever any hot hatch came sniffing, know that I could leave it choking in my dust. I’d probably never exercise the option but it would amuse me to know it was there.

Make no mistake, Mercedes has turned the E 500 from a respectable effort into a memorably fine machine. I just wonder how many potential customers will walk straight past their dealership without realising.

THE OPPOSITION

Model BMW 550i SE £44,110
For Very sporting to drive with strong performance
Against Looks odd, stiff ride, nightmarish iDrive system

Model Audi A6 4.2 FSI quattro SE £44,475
For Good looking inside and out, quiet and refined
Against Limited performance in this company, dull handling