Road Test: Mercedes-Benz GL Class GL350 BlueTEC AMG Sport 5dr Tip Auto

Are you the sort of well-heeled parent who, having already spawned four children, gazed into the eyes of your spouse and said: "One more, and we'd have enough for a five-a-side football team"? If so, pray point yourself towards the cavernous enormity of the all-new Mercedes GL.

This is one supersize SUV: longer and taller than Audi's Q7, a seven-seat big brother to the not-very-small Mercedes ML. Such enormity makes it very popular Stateside: Merc claims the last GL was the USA's best-selling large luxury SUV, and the new one should go even greater guns.

We Brits are a more discerning breed. For a start, we won't even countenance a big SUV unless it's diesel. When the GL reaches the UK early next year, there will be a petrol option (the 435bhp GL500, a twin-turbo V8), but all but a handful of Brits will opt for the diesel GL350: 254bhp, 372lb ft and a theoretical 38mpg.

Piped through an unhurried seven-speed auto 'box, the 3.0-litre V6 diesel is smooth and strong but hardly an engine to put the ‘Sport' into ‘Sport Utility Vehicle'. The GL doesn't chase the (admittedly smaller) Porsche Cayenne or BMW X6 down the look-at-me-handling-like-a-saloon route.

Even with active chassis switched to Sport, this remains a big wafty bus, isolating you from the road beneath rather than pressing your face to it. Any attempt at right-foot foolishness induces stroppy electronic flashing, the GL far happier to potter gently, absorbing bad roads with unGermanic compliance.

Isolation is what the GL's about, featuring - and I quote - Acoustic Front Windows, Sprayable Acoustic Materials and Optimized Aeroacoustics (which sounds like the worst three-band folk night ever). It's whisper-quiet, and, if you get liberal with the options, dripping with top-end technologies, including a 360-degree parking camera and active tilt stabilisation. Specced up, the GL feels fresher and posher than Audi's ageing Q7.

But there's a big British fly in the GL's ointment: our very own Range Rover, which does the effortless 4x4 luxury thing with unparalleled panache. And there's an all-new version on the way to raise the bar yet higher. If you can cope without the third row of seats (does your five-a-side team really need a defence?), take a drive in the new RR before committing to a GL.

But, if seven chairs are a must, the Merc is a refined, if stolid, choice. Stolid, that is, unless you chose the upcoming 557bhp GL63 AMG, capable of 0-62mph in less than five seconds: a daft figure for a 2.5-tonne mega-SUV. Unless you need to get that team to an away fixture in a hurry...