Spending a six-figure sum on a car is not a rational idea. An S-class Mercedes offers all the comfort, pace and prestige anyone could need for ‘only’ £70k, so spending more is always going to be hard to justify. But even in these straitened times there are still a few car buyers who are prepared to leave logic out of their buying decision, enough to keep bespoke motor car makers like Bentley in business.

The four-door Flying Spur, the Crewe company’s closest competitor for a big Merc, starts at £120,700 and for those who subscribe to the idea that too much is never enough there’s this: the £137,000 Flying Spur Speed.

The unsubtle name – it harks back to the ‘Speed model’ Bentleys of the 1920s – denotes more power and greater straight-line performance than the standard car, together with a package of upgrades designed to sharpen its responses.

The suspension is lower and stiffer, the steering has been recalibrated and the wheels are now larger and shod with bespoke Pirelli tyres. The ESP system, an electronic watchdog which senses a skid and applies the brakes on individual wheels to bring the car back under control, has been re-tuned to operate more progressively.

Power still comes from Bentley’s twin-turbocharged ‘W12’ engine which has 12 cylinders arranged in a pair of Vs, but the Flying Spur Speed has 600bhp available, up 48bhp on the standard Flying Spur. That’s four or five times as much as the average saloon car. The Flying Spur Speed is the most powerful Bentley saloon to date – and on the road it shows.

Though the Spur’s sheer speed is amazing, it’s the effortless manner in which it performs which is truly impressive. Demonic acceleration is available with a flex of the right ankle, while no more than a discreet rumble permeates the calm of the cabin. No need for disturbing sounds of thrashing machinery or screaming superchargers here.

Left to its own devices the gearbox is all but unnoticeable, slurring effortlessly between ratios, but if you feel the need you can take control using a pair of levers mounted behind the steering wheel. They work well enough, but it’s odd that they’re made out of rather low-rent black plastic when the interior is dominated by wood, leather and chrome.

Four-wheel drive ensures that wheelspin is never an issue, and it makes the Bentley’s enormous power far more usable. Twisty roads are tackled with a composure rare for such a large machine, though the ride quality lets the side down. It’s leagues ahead of most cars but lacks the silken cushioning of some luxury saloon rivals, not least an S-class Merc.

What few cars can match is the sense of occasion which starts the moment you reach for the massive, chromed and Bentley-badged key. A Bentley may not be substantially quicker, quieter or comfier than a bunch of other big saloons, but it does feel special. The imposing size, the crushing performance and an interior that is a hand-built work of art all contribute, and that’s what Crewe’s customers pay extra for.

Despite the worries of the world’s financial markets – and the shrinking investment portfolio of many a would-be Bentley buyer – the company is pressing ahead with new models. Later this year the new Mulsanne limo arrives to take on Rolls-Royce. At the Goodwood Festival of Speed last year Bentley unveiled a Supersports version of the Continental GT, the Flying Spur’s coupé brother, with an even more powerful engine and supercar-class performance.

A Superports version of the Flying Spur might be next – if Bentley can find enough irrational people to buy it.

2009 Bentley Continental Flying Spur Speed specifications

6.0-litre W12 twin turbo petrol engine, 600bhp @ 6000rpm. Six-speed automatic gearbox, four-wheel drive. Four-link front suspension, multi-link rear with air springs. Dimensions 5290 x 1916mm. Maximum speed 200mph, 0-60mph 4.5 seconds. Combined cycle fuel consumption 17mpg. CO2 396g/km. Price £137,000

www.bentleymotors.com