YOU might think this slightly mad Audi is an answer to a question nobody asked - but you might still be glad you could buy one.

Slightly mad? Well, it seats seven, will venture off road if needed but accelerates like a lurcher escaping a horde of demented hornets.

It's so fast you will constantly see naughty numbers on the speedometer with the merest flex of a right foot.

So fast, you'll actually laugh out loud at the way this two tonne (and some) leviathan gathers pace.

Audi says this £70,970 SQ7 is the UK's fastest, most powerful diesel SUV and they're surely right. It makes the most brutal offerings from the likes of Range Rover and BMW look a little feebly-muscled by comparison.

Top speed is electronically limited to 155mph (goodness knows how fast this bluff prowed powerhouse would go in unrestricted form) and it claws its way to 62mph in 4.9 seconds.

That sounds pretty spectacular but you have to experience the way it gathers speed to turn simple paper figures into something just this side of a white knuckle fairground ride.

A ride that's actually more civilised than first feared (Audi has solid-springs form with its fastest vehicles), especially if you find another £5,700 for the driving dynamics sports pack which uses electronics to uncouple the sway bar on uneven surfaces that stops the car leaning so far in corners when you're pushing on a bit.

That needs an instant response and that means a 48 volt electrical subsystem with its own high-tech battery in the boot.

The same power source lets the SQ7 become the first production car in the world to boost engine response with an electrically powered compressor.

This little turbine blows instant air into the engine even before the two turbochargers wind themselves up - helping cut the lag between right foot prod and on-road reaction to undetectable.

The result of all this puffing and huffing is 435 horsepower and so much pulling power (an extraordinary 900Nm for the technically minded) that simply renders the 2,330kg of an unladen SQ7 about as relevant to forward progress as a keep out sign to a charging rhino.

And it's all accompanied by a sonorous thunder that only accompanies a proper V8 engine, which you'd never believe was a diesel in this application unless a look at the rev counter gives the lower-revving game away.

All this power and heft comes at a cost, of course. The SQ7 will quickly learn its way to the pumps, with only the most severe restraint edging consumption over 30mpg. A test run on typical UK roads showed 29.4mpg and the official average of 39.2mpg will forever produce a rueful smile.

Not that the buyer of Audi's flagship SUV is going to worry about economy. There's the new £64,950 Q7 e-tron diesel plug in electric hybrid for that, with the possibility of 34 miles silent progress before the diesel cuts in.

Silence and economy are rather in the background for the SQ7, although both versions share Audi's winning obsession with cockpits built like the boardroom of the world's most prestigious architectural practice - all soft leathers and so, so tasteful alloy highlights.

Combine the loving feeling this produces with a list of standard equipment that looks on the generous side for an Audi and the SQ7 doesn't look that dear, although the test car showed how too much ticking of options' boxes mounts up.

The bottom line came to an extraordinary £95,160 and included such niceties as a £1,100 Bose sound system, glass sunroof (£1,700), head up display (£1,350 and excellent) and the already mentioned £5,700 driving dynamics pack, which everyone will want.

But you might look at a heavily optioned SQ7 as a bit of a bargain. No really; its underpinnings are already to be found in the new Bentley Bentayga, which costs from £160,200 before an owner also succumbs to the options list seduction.

The Bentley currently offers only a vast petrol engine but there's a diesel on the way. And yes, it will be based (very closely indeed) on the one you've been reading about.

So find the £70,970 needed for the fleetest of foot Audi SUV and enjoy your half price Bentley.