IT'S the forgotten princess of BMW's range.

Elegant, immodestly large, serenely comfortable and surprisingly rapid, the current 6 Series convertible has been with us for six years now which makes it the marque's longest running unchanged model.

Despite its substantial size, there's comfortable room for just two onboard, unless you are content to crouch down and fold yourself into the small rear seats.

This is, after all, a grand tourer with oodles of power an ample boot space for your matching designer luggage.

There's a choice of a couple petrol models and a diesel version but the smart money is on the 640d powered by a turbocharged oil-burner which has enough thrust to satisfy most, yet makes more sense both tax and economy wise.

The extrovert open-top maybe aimed at those who put boulevarding before blasting along mountain passes but that doesn't mean it can't satisfy the keen driver.

A squat stance and good weight balance help the 640d provide a delicacy of handling matched by only the best performance cars.

Sure it's big and heavy but the ride is excellent and through the steering you feel what's going on at road level. Roll is slight and there's little sign of any lack of rigidity because of the absence of a steel roof.

Its 3.0-litre turbo six-cylinder diesel endows it with 313bhp which is enough to trim the 62mph sprint down to a tidy 5.5 seconds which is seriously fast.

There's little drama to proceedings as it hurtles serenely on to 100mph in around 13 seconds and is prematurely cut short at 155mph by its electronic limiter.

An eight-speed automatic gearbox complete with steering wheel paddle changes is as unflustered and silky as the power unit itself and proves the ideal complement to a relaxed though brisk long distance express.

The cabin, decked out in sombre but high quality leather, is smart but starting to look somewhat dated. There's reasonable storage space by way on central locker, door pockets and passenger glove box, plus the back seats are useful for stowing coats etc when not being perched upon.

The boot itself is shallow with a narrow aperture but large enough - 350 litres with hood up and 300 litres when it's folded - for weekends away.

Folding is a quick and easy business - just a press of a button. When the hood is in place, rear and three-quarters vision is pretty poor because of the wide fabric pillars but once retracted the problem vanishes.

You expect a fair amount of goodies for a car of this price, and the 640d M Sport gets most of them including, sat nav, climate control, heated seats, auto lights and wipers and full leather trim. The M Sport pack is largely cosmetic tweaks.

Few 155mph luxury cars are capable of topping 40mpg, but the 640d, despite its thundering pace, is one of them. My average was 37mpg with a best of 43mpg.