How does a car end up costing £850,000? Well, the man looking after this Bentley GTZ helpfully offered the fact that the fantastically detailed LED tail-lights cost £79,620 a pair.

This special Bentley is the embodiment of “no expense spared”. It is essentially a Continental GT Speed - itself a hugely excessive car and the first Bentley to be officially capable of topping 200mph - which has been heavily customised by Zagato, a design and engineering consultancy based in Milan. Zagato was founded in 1919 and has produced stunning special edition coachwork for a who’s who of manufacturers, including Ferrari, Aston Martin, Alfa Romeo, Maserati and now Bentley.

Having taken delivery of a GT Speed, Zagato strips the bodywork and interior. A new body is hand-beaten in aluminium, and the interior is retrimmed with Zagato’s chosen materials. It can be tailored for the owner in the way you might have a Savile Row suit run up. Want a shade of paint to match your wife’s (or husband’s) eyes? No problem.

The idea for this rolling monument to wealth came from Franz-Josef Paefgen, the boss of Bentley. He’d seen a collection of historical Zagatos at a concours d’elegance in California and been so taken with the grand style that he invited Zagato to try its hand with a GT Speed. Only nine of these special-bodied Bentleys will be offered for sale.

Zagato doesn’t touch the engine or mechanicals of the car, and each example must be signed off by Bentley. That means everything in terms of fit and finish must be up to factory standard, which is no mean feat for a hand-built car.

It looks impressive. That sloping double-bubble roofline is a signature piece of Zagato design - it also presents a logistical problem because the double bubble extends to the rear window, which must be formed in a wave shape. That’s the sort of problem where “cost no object” comes into play.

From the front, the GTZ is more aggressive looking than the standard Bentley. The grille has been enlarged and the spoiler is lower, creating a visual impact that makes the standard car look almost apologetic by comparison. The GTZ’s bonnet also sits a bit higher and houses functional air vents.

The interior, though, looks much more like a standard Bentley - wholesale changes to the architecture would be too difficult in an age of airbags and finely tuned crash structures. There are, though, some delicious Zagato touches, such as the intricately stitched leather roof lining that feels so marvellously louche. Less successful is the sparkly green paint on dash surfaces - it looks like something you’d get at Halfords.

The view over that massive bonnet is fairly conventional, but glance in the rear-view mirror and you’ll have to stifle a laugh. The rear window is so shallow that it is like looking through a letter box. But, as I learn, what’s behind you isn’t important.

The GTZ produces a colossal 600bhp and 553 lb ft of torque, courtesy of its W12 engine. And because this special edition is a little lighter than the regular Bentley, performance should be pulse quickening.

Floor the throttle for the first time and a better description of the GTZ’s acceleration would be heart-attack inducing. Bentley claims a GT Speed will reach 60mph in 4.3sec and won’t stop until it hits 202mph - the GTZ feels quicker.

The noise is suitably titanic, too. The GTZ’s exhaust has been tuned to be a bit more raucous. Imagine an upset grizzly bear and you’ve got it. Even better, when you lift off the throttle suddenly, there’s a rolling thunder of pops and rumbles, which is fantastic.

To judge whether the Bentley GT Speed had lost anything in its translation into a Zagato, I drove a Speed back to back with the GTZ. And apart from a slightly more pedestrian exhaust note (and better rearward visibility) in the standard car, the experience was identical. Except for one thing - a Bentley GT Speed and the standard Bentley Continental GT are a fairly common sight in the south of England, but drive the GTZ and you’d better get used to other drivers diving in for a closer look.

The price is still faintly ludicrous: for £850,000 you could buy six Bentley GT Speeds and have enough left over for a VW Golf GTI. And I don’t want to think what it would cost to insure. The GTZ is a slice of glorious madness that confirms what we all know: the rich aren’t like us.

- A right-hand-drive GTZ is in the pipeline, but it won’t be finished until a buyer is found, who will then specify colours and trim materials

- The wavy rear window looks novel but tends to warp the view out of the back The interior uses the Continental GT Speed’s switchgear but is retrimmed and reupholstered by the Italian firm Zagato

- Every body panel, including the wings, has been restyled by Zagato and hand-beaten in aluminium

- The tail-lights cost a staggering £79,620 for the pair - good thing the GTZ has parking radar

Hot Wheels specs

MODEL Bentley GTZ
ENGINE 5998cc, V12
POWER 600bhp @ 6000rpm
TORQUE 533 lb ft @ 1750rpm
TRANSMISSION Six-speed automatic
FUEL/CO2 17mpg (combined cycle) / 396g/km
ACCELERATION 0-60mph: 4.3sec
TOP SPEED 202mph
PRICE £850,000
ROAD TAX BAND G (£400 a year)
VERDICT It doesn’t get more exclusive - or more expensive - than this