If logic had a great deal to do with the product planning process, the Mercedes SL600 would never have lumbered from the drawing board. When you've already got one 5.5-litre 388bhp SL model that accelerates to 60mph in 5.4 seconds and on to an electronically limited top speed of 155mph, why on earth would you need to introduce anything faster? The answer seems to be because Mercedes can.

Of course, at this end of the market, customers are accustomed to getting what they want. Moreover, Mercedes have a twin-turbocharged 5.5-litre V12 that they were unable to resist fitting to an SL body. Priced at £101,265, the SL600 is, in the light of its achievements, still something of a bargain, especially when compared to cars like the Aston Martin Vanquish, the Ferrari 599GTB and the Lamborghini Murcielago, all of which would struggle to keep up and none of which can hold a candle to the mighty Mercedes' 830Nm of torque. The power of this engine has now been massaged up to a massive 517bhp, usefully more than the 388bhp boasted by the V8-powered SL500, the next model down.

The interior of the SL600 has been appointed with a few more luxury features too, the SL's embossed Nappa leather chairs incorporating massaging multi-contour backrests. The steering wheel and gearstick are trimmed in wood and leather and the COMAND information system has a built-in TV tuner. A mobile hands free telephone is concealed in the centre armrest and sound comes courtesy of a very punchy Bose speaker system.

Externally, the SL600 can be identified by its massive 18-inch alloy wheels and the V12 badging tucked into the side air intakes. Chrome trim around the radiator and silver brake calipers also mark out this behemoth. It also feels markedly different as soon as you start the engine. It stays muted until around 3,000rpm when it starts to charge and by 5,000rpm it's emitting quite a bellow. The twin turbochargers seems to have an 'always on' quality about them which means that you're never caught out waiting for them to spool up to speed. Sixty from rest is just 4.5s away, a second quicker than the SL500 can manage.

"Mercedes claim the SL600 is a responsible supercar?.."

The addition of the huge twelve-cylinder engine and the additional luxury accoutrements add up to an awful lot of weight but even so, the SL600 never has that hulking nose-heavy feel tipping into a corner that affects many front-engined V12 cars. The transmission offers three modes of operation - comfort, sport and manual. Switch to manual and you get to play with steering wheel mounted shift buttons, but the shifts are never quite as crisp as a proper semi-automatic manual gearbox. But them, with a car that makes everything seem so easy, perhaps riflecrack gearchanges would be a little out of character.

The roof mechanism has stepped forward a generation from that of the SLK, lifting or lowering in a mere 16 seconds. Whereas the SLK sacrificed its boot in order to accommodate the flashy Vario roof, the SL600 is cleverer. When opened or closed, the rear screen rotates to reduce the amount of boot space the folded roof impinges upon. Still, the 11 hydraulic cylinders utilised help the SL600 to a hefty kerb weight of 2044kg; a fair bit heavier than even a long-wheelbase S-class limousine.

Although there wasn't much wrong with the SL's basic shape, a design that has endured extremely well from its 2002 launch, the engineers felt that the driving dynamics and detailing were in danger of falling off the pace and have made many changes to the latest SL, not all of them immediately obvious to the casual observer. Let's start with the bits you can see. The front end has been redesigned, although it helps to have the old and new cars side by side to see exactly where the nips have been nipped and the tucks been tucked. The front bumper assembly has been revised with three large cooling air intakes integrated, giving it a more aggressive V-shaped appearance. The fog lights have also been repositioned with chrome surrounds, visually lowering and widening the front of the car. A matt silver finished grille with three broad slats and chromed headlights complete the changes up front, while at the side there's a wider choice of alloy wheel designs. The rear view shows a redesigned light cluster with horizontal divisions between red and white clear lens sections.

The interior has also been revised with a commitment to offering the sort of quality that buyers in this sector expect. So it is that Mercedes have fitted even softer leather for the upholstery, revised the interior colour palette, added high quality embossed metal doorsills and a redesigned chronometer style instrument pack. A remote bootlid release system is included for the first time and aluminium-look switches for opening the roof.

A developed version of the ABC (Active Body Control) system is of course standard. Compared with the old ABC system, body movements in dynamic driving situations are claimed to be reduced by up to 60 per cent, offering a flatter, more composed ride. ABC has the wonderful effect of making a very big and heavy car feel light and 'chuckable'. Re-geared steering and punchier brakes also boost the SL's appeal as a serious driving tool.

The ABC system has been designed to interface more efficiently with the ESP stability control system to avoid the stRangely artificial feel when cornering hard that afflicted earlier incarnations of this technology. Turn the ESP off and an SL will still display its lairy side. If you can overcome the mental block about treating a super premium roadster like a Mazda MX-5, the SL is extremely capable and, dare we say, fun. In the past, most sporting Mercedes models seemed to wilfully introduce impediments to the process of enjoyment, either lazy automatic gearboxes, laughably poor manual gearchanges, vague steering via steering wheels that resembled something from the Cutty Sark or woolly chassis. It appears the killjoys never got their hands on this SL's design.

Mercedes claim the SL600 is a responsible supercar, returning a relatively impressive 19.6mpg on the combined cycle and emitting less C02 than a V6 Jeep Cherokee. Despite its comparatively clean social conscience and bargain price, this is a car with supercar sized running costs. Some may see it as the superfluous supercar but the motoring world would be a poorer place without cars as magnificent as the SL600. Logic, after all, would probably drive a Kia.