In today’s market, you’re no-one if you don’t offer a plutobarge with a big diesel engine.

The Mercedes S320 CDI has been available for some time now, although it’s a shame we don’t get the more interesting 246bhp S400 CDI that’s available elsewhere in Europe. After taking a while to realise the party had started without them, BMW now offer the 7-series in 3.0-litre diesel form.

But with Jaguar and Lexus pretending their invites to the diesel party were lost in the post, it’s left to Audi to prove that tractor fuel is where it’s at.

Those looking to burn oil will find two models of interest in Audi’s A8 range – the six-cylinder 3.0 TDI and this, the eight-pot 4.0 TDI.

In the power wars, the A8 wins. Its twin-turbocharged 4.0-litre V8 pushes out 270bhp and 479lb/ft of torque at 1,800 rpm, figures beaten only by VW’s 5.0-litre V10, but still easily enough to squirt the aluminium-bodied Audi to 62mph in 6.7 seconds and on to an electronically-limited 155mph.

There’s a space/time-bending quality to the power delivery, particularly when flooring it from a stand-still. There’s a brief delay as the turbos spin up, followed by an almighty shove that flattens you into the seat and transports you seemingly instantaneously to a point 500 yards further down the road. It’s as if the car remains stationary while the engine’s huge torque spins the planet underneath it. For those two or three seconds of initial acceleration, you feel like you’re driving a car powered by God himself.

It’s an aurally pleasing experience, too. Despite some clatter on initial start-up, this is a diesel V8 that woofles like a petrol. At low revs, the 4.0-litre is reminiscent of a 1960’s American muscle car and higher up the rev range things stay calm while developing a harder edge.

The six-speed Tiptronic transmission is generally effective at making the right selection of cogs, and does so without fuss or much in the way of delay. It’s eager to kick-down, too, rendering the manual selection of gears using the Tiptronic function almost unnecessary.

Power is sent through all four wheels via Audi’s now customary quattro system. Four wheel-drive is considered mandatory for any car with this level of power, and the system copes well with the task of distributing the engine’s gargantuan torque.

The standard air suspension, however, doesn’t do such a good job.

The all-aluminium body construction keeps weight down and helps the suspension make a decent fist of body control, with roll and tip-in kept well in check. But it’s the A8’s ride quality that really lets the side down.

Small bumps and imperfections slip easily through the air suspension’s defences and make their presence known with a thump in the cabin. Cat’s eyes and tarmac ridges that would be well suppressed and smothered in the A8’s competitors become a jarring annoyance in the Audi. The system features several modes of operation (comfort, dynamic, etc) but the differences between them are too subtle and do little to improve the overall quality.

The steering, while beautifully light at parking speeds, weights up artificially with more than a quarter turn of lock, disguising what little feedback there is.

Take a moment to look around you in the cabin, and the disappointment might start to spread. Many of the controls will be familiar to anyone who’s sat in any recent Audi, and that spoils the atmosphere somewhat. Buttons and switches feel lose and rattly, the screen for the MMI (Multi Media Interface) system appears out of the centre of the dash in a wobbly, crunchy and graunching motion and the door bins have a particularly cheap feel. It certainly doesn’t feel like a £60,000 luxury car.

We’ve criticised Audi’s MMI before, but it’s so important we’ll do it again: in a car likely to be driven by a 60-year old technophobe, these systems are a disaster. While Audi’s MMI isn’t quite as obstructive as BMW’s iDrive, this Germanic obsession with overly complicated control systems really ought to stop.

Interior space isn’t even that wonderful. The cabin doesn’t feel much wider than that of the A6, although there’s undoubtedly greater headroom and more space in the back for your golfing partners. And, as usual for Audi, the electrically adjustable steering column doesn’t adjust low enough.

Equipment levels are as you’d expect, but no more. You’ll find 14-way electrically adjustable front seats with memory, cruise control, nine-speaker stereo with six-disc CD changer, 18″ alloy wheels and parking radar.

List price for the A8 4.0 TDI quattro is £58,600 on-the-road, to which we added Advanced Key (£850), automatically actuated boot lid (£375), Xenon Plus headlights (£500) and DVD based navigation system (£1000) bringing the total to £61,325. That’s around £8,000 more than a Mercedes S320 CDI and a frankly unbelievable £11,600 more than a BMW 730d SE.

Be prepared to lose much of that value, with only 45% of it retained after three years. Insurance is high, too, at group 19, and servicing isn’t what we’d call value for money. The level of customer service from most parts of the Audi network is good, however, with appointed A8 Specialists available to help you at any time (perhaps a reflection of this car’s unnecessary complexity). Government figures reveal a 28.8 mpg thirst on the combined cycle, and although we achieved much less than that during our test, we’d expect to improve upon that figure on a long journey.

So that leaves us with an over-priced, over-complicated saloon with questionable interior quality and a poor ride. Its one saving grace is the fantastically powerful diesel engine.

And what an engine.