Road Test: Mercedes-Benz M Class ML420 CDI SE 5dr Tip Auto

I used to love the fact the old M-Class was so heroically rubbish - that it used to lull so many badge snobs into a false sense of smugness, completely unaware that their 'status' SUV was actually a complete stinker.

Never has there been a car that so neatly articulated just how dangerous an imbalanced money:sense ratio can be.

But that's all changed with the new M. Mercedes gave those poor misguided egos a break by building a car that better deserved its premium brand; or, looking at it another way, it was built properly. And now Merc's added a diesel engine that trumps the old ML 55 AMG.

This then, to those that haven't blindly gone ahead and ordered one, is the new ML 420 CDI. Its 4.0-litre twin-turbo V8 serves up the same power as the current ML 500's petrol V8, but has 50 per cent more torque - that's 517lb ft. There's a bit of deep rumble that finds its way into the cabin if you give it a real bootful, but it brings proper, usable shove that arrives at a lowly 2,000rpm and, until you get used to it at least, will keep surprising you.

As will the performance figures; zero to 62mph in less than seven seconds and a top speed of 146mph. Both stats are better than the petrol V8's and the diesel still gives you five extra miles for each gallon of fuel.

Add the fact that a 420 only costs an extra grand and the ML 500 is dead in the water.

And that is probably what's behind Mercedes' thinking in bringing this car to the UK. Big V8 diesels have become something of a status thing in Germany and with 60 per cent more oil burners sold over here than petrol models, torque-hungry buyers will find the 420 a no-brainer in the same way.

It gets better, too, for Mr Smug. Along with the quality upgrade, the new M-Class actually rides and handles well enough to make you think twice about almost any other rival. In fact, if you cop one of those dismissive, down-the-nose glances in anything less than a Range Rover, you're missing the point.

But before you rush out and drop nearly £45k on an ML 420, a word to the wise. If you can put up with impressive instead of monumental torque, consider saving yourself 10 grand and buy the 320 CDI V6 instead. Either way, don't buy a petrol-powered M - leave that to the clueless and give them a little knowing smile as they rumble past.