THERE was a time when a true grand tourer was an elegant cabriolet with flowing lines, generous boot and mountainous performance.

Well, the good news is that nothing has changed. For BMW, the formula of understated power remains the same in the latest 8 Series range as it was decades ago in the 6 Series Coupe.

Further enhancing the glamorous trans-Europe capability of the 840 Convertible I've been trying out is the fact that it's a diesel. Sure, the purists will moan that it doesn't sound quite right and the red-line on the rev-counter is too low.

But the reality is the big turbo diesel can cross most of France between fuel stops. And it sips the costly liquid rather than gulping it. Surely a useful advantage when touring.

The 318bhp, three-litre, six cylinder diesel may not deliver the aural thrill of a V8 petrol but there's no doubting its clout and mid-range grunt. Floor the accelerator and 62mph flashes by in under five seconds.

The gravelly roar of the twin scroll turbos is a reasonable substitute for petrol snarl, and when full power isn't being applied - ie much of the time - there's little more than a murmur coming from the long nose.

Ensuring that all the horses keep their hooves firmly on the ground is xDrive four wheel drive. Not only does this aid cornering and help in slippery conditions, but it prevents embarrassing moments of wheel spin during fast getaways from standstill.

An eight-speed automatic gearbox by ZF ensures an appropriate balance between refinement and serious shove. Steering wheel paddles allow you a tad more control and can be fun on the right roads. They are particularly useful for a nifty overtaking manoeuvre.

The suspension is a tour de force allowing a composed and almost soft ride with negligible roll. Adjustable dampers are standard as is active steering which sharpens the response and allows decent communication with the road surface.

Weighing in at nearly two tons, it's a big car and although the power and dynamic ability tends to counteract its girth, the centrifugal forces of physics are not to be totally denied. Nevertheless cornering can be rapid and secure.

The cabin is sensible and solid rather than extravagant but the use of quality hide and soft touch materials remind you this is more luxury tourer than sports car. Digital instrumentation, Cockpit Professional and the newest version of BMW's iDrive are there to be enjoyed.

I'm not so sure, however, that the hewn glass gear selector - which costs an extra £575 - is quite appropriate. More Vegas than Verona perhaps...

Space two-up is more than ample but if four pile-in, those in the back had better have short legs because this is strictly a two-plus-two. The electric fabric hood goes down in a jiffy but there's a fair bit of wind turbulence experienced, particularly for those in the back.

Few would expect a 155mph luxury express to attain nearly 40mpg, but that's exactly what the 840d is capable of even when delivering high average speeds.