BMW is most readily associated with sleek, smooth saloons that drive like sportscars, but over the years it has quietly accrued a pedigree for classy convertibles, too.

From the original 328 of the 1930s through the 507 Roadster of the 1950s and the original 6 Series, the company's back catalogue is impressive.

The Z4 marked a reawakening of serious droptop intent on BMW's part after the slightly disappointing Z3 and rather too rare M3 Convertible. But if the Z4 can be described as dramatic, the 6 Series Convertible goes much further as nothing short of epic.

All the main requirements are here - power, beauty and intrigue, on a truly Trojan scale.

Blessed with the sort of heroic muscularity usually reserved for classical sculptures of Hector, the 6 Series also boasts hidden electronic wisdom worthy of Cassandra the prophetess, to get you out of any sticky situation short of a sevenyear siege. And it can charge forward with earthscorching ferocity thanks to the 333bhp from the V8 engine. Who needs Hoplites when you have a whole cavalry division at your disposal?

Arch-rival Mercedes is well known for pioneering the folding steel roof. Other carmakers, from Peugeot to Renault, have followed suit. But, with typical selfassurance, a BMW spokesman described the trend as a "fad". That's not meant to imply that the others are wrong, merely that it's not the BMW way of doing things.

The 645Ci has a "conventional" folding roof, even if the multilayered design and sophisticated folding mechanism seem anything but ordinary. It operates in around 20 seconds though its most impressive attribute is its ability to dampen sound.

Developed alongside the 6 Series Coup, a decision was made that the 645Ci had to share the same coup profile with the roof up. That may not sound like much, but it is a technical triumph for a convertible, particularly as BMW's trademark 50/50 weight distribution also remains intact. A pair of rakish shark fins at the back of the hood instantly set the car apart from its competitors.

At the front, a broad, lowlevel grille and headlamps like wraparound shades make this the bestlooking modern BMW by some distance. More graceful than a 5 or 7 Series, it's also more fluid than a Z4.

Because the convertible is very nearly as rigid as the coup, it's even more involving to drive than its twin when the roof is down. Large convertibles can easily feel too "rangy" - great at cruising in straight lines, less impressive around tight bends. This car feels at ease in both situations, only betraying the underlying instability of a long wheelbase, heavy drop top when the driver indulges in a show of hubris by switching off the traction control system.

Though officially described as a "2+2 convertible" (meaning the front two seats are fullsized with smaller part-time rear seats), BMW is showing uncharacteristic modesty. Rear space is actually very good and even a longdistance journey wouldn't be too irksome - particularly because, in a 645Ci, a long distance is unlikely to take a long time. Without wanting to encourage irresponsible behaviour, it's fair to say that this is not a car that encourages the driver to merely amble along.

All 6 Series models come with the revised iDrive infotainment system fitted. It's easier to use than it was in its early days, but still annoying and liable to be unwittingly altered by the front passenger thanks to its poor location next to the handbrake.

The boot is surprisingly roomy - a clever level even allows it to be expanded by a suitcaseful when the roof is up.

Describing how the car drives is almost superfluous. It does everything you'd expect of a big, expensive car with a big, powerful motor from a big manufacturer with a reputation for building outstanding drivers' vehicles.

A 4.4litre V8 engine provides plenty of motive force and one of the most alluring soundtracks of any modern motor.

It was international engine of the year in 2002 and it's not as much of a gasguzzler as you might imagine - when mated to the sixspeed automatic gearboxes (either with Steptronic manual selection or the sequential paddleshift SMG system) it actually returns better fuel economy than the manual box.

Standard equipment includes leather trim, dynamic stability control (DSC), antilock brakes (ABS) with cornering brake control (CBC), dynamic brake control (DBC) and electronic brakeforce distribution (EBD).

If you want heated seats, cruise control or the Active Steering that BMW makes such a fuss about, you'll have to plunder your personal treasury. Another likely addition is a wind deflector - the car's rear window (which operates independently of the hood) is fairly ineffective at stopping buffeting.

BMW's claim that the £55,000 plus price tag "could be called a bargain" may be a little tongueincheek but it does point up the fact that anyone in a position to buy a 645Ci will find it a reliable investment.

For more information, call Woods BMW on 01256 703703.