SO what's the world's best selling luxury SUV?

If you opt for Range Rover, as I was tempted to, there are no prizes. Similar story with the legendary Toyota Land Cruiser. The right answer is Mercedes-Benz M-Class which has clocked up 1.2 million sales in the last 15 years.

The latest version is already building on that success and for the first time there's a four cylinder diesel model in the line-up lending extra economy as well as a pared down list price, but with a start price of £45,380 it's hardly low rent.

Amazingly the big off-roader which is a full 4.8 metres long and stands as tall as most men (up to the roof rails), is powered by a modest 2.1-litre four cylinder diesel engine. But it belts out a lusty 204bhp which, only a few years ago, would have been considered a good result from a petrol-powered hot hatch.

It's a bit less silky than its six cylinder brethren but there can be no doubting the pace and efficiency.

A seven-speed automatic box slings the tall shape forward despatching 62mph in a swift nine seconds and it finally runs out of steam at 130mph.

All this and an average economy of around 35-plus mpg (official combined figure is 44.8mpg) can't be bad. Its driving character is more luxury than sporty and the ride therefore is softer and more absorbent than say a Range Rover Sport or BMW X5 and you are aware that the forces of gravity are at work when cornering quickly.

It's a big car inside as well as outside. Ultimate cargo carrying capacity is more than 2,000 litres with rear seats folded down and even with them in place there's room for 690 litres of luggage putting it among the most commodious SUVs.

There are stacks of flat surfaces, cubbies and pockets to stow oddments and the cabin architecture bears all the hallmarks of a luxury saloon. Robust, heavy duty plastic mouldings are totally in keeping with the M-Class's tough, no-nonsense reputation of longevity, if a bit on the sombre side.

There's a clever touch to the front cup holders - by tapping a button your drink can be chilled down or heated up.

Despite the slightly gruff note at start up - a diesel telltale sign - the ML 250 BlueTEC Sport quietens down nicely on the move and compares well with a luxury saloon in terms of refinement.

The extra height and good visibility allows high average speeds to be kept us as the driver can easily see over hedge tops or the roofs of other cars.

Inevitably, there are stream of tempting extras that can be added to the already reasonable standard spec.

This car came with a sat nav and media sytem at £2,130, glass panoramic sliding sunroof (£1,835) and ‘memory package' - no jokes please - which includes electrically adjustable head restraints, seats and steering column with automatic recall.