Volkswagen EcoRacer

In fact, it's hard to know where to start with this innovative two-seater coupé. The EcoRacer's bodywork and chassis are made of carbon fibre reinforced plastic (CFP) - VW calls it a 'polycoque'. Rust-proof, strong and recyclable, CFP is more expensive than steel, but is perfect for limited-run models such as the EcoRacer.

VW's engineers are trying to persuade management to give the sports car the green light. "We have a problem with limited production runs," said one. "We don't think about building 5,000 vehicles a year; more like 5,000 a day. It would be good to change that."

The EcoRacer's styling is a mix of retro and ultra-modern. There's a touch of the old Porsche 914 from the Seventies - a favourite of designer Cesar Muntada. But the mid-engined VW is smarter than that, and the versatility which is demanded by today's buyers is met comfortably. The entire T-bar roof can be removed and stored behind the rear seats, which converts the EcoRacer into a roadster. And the top part of the dashboard and windscreen can also be swapped for a minimal wrap-round aero screen, thus transforming the car into a speedster.

Under the engine cover is perhaps the biggest surprise: a turbodiesel powerplant. We've seen oil-burning hot hatches and cabriolets before, but this is the first diesel sports coupé.

The 1.5-litre unit produces 134bhp and a blistering 249Nm of torque. It is also one of a new breed of VW engines that uses hi-tech internals, and the firm is promising that we'll see these innovations on the road within five years. Power is fed to the rear wheels via the smooth-shifting DSG sequential manual transmission.

Our test drive took place at the Pau circuit in southern France. The roof flips up to allow easier access to the cabin over the wide sills, and you sit in a bare carbon fibre cockpit in a comfortable and supportive bucket seat.

The hi-tech, blue-lit instrument binnacle has read-outs for speed, fuel range and water temperature, but that automatically changes when you select race mode, which displays engine revs, turbo boost and oil temperature.

Switches and instruments come mainly from the Golf and Passat, which also donate most of the suspension, brakes and electronics. It's a lovely looking dashboard, and in virtually all respects the EcoRacer feels like a well prepared track model - apart from the diesel rattle behind you. Pull away, and the performance is startling. It's a classic mid-engined sports car, and the steering wheel is alive with information.

The engine's powerband is narrow, so you need to use the gearchange paddles on the wheel to keep it on song. It also runs out of revs at 4,500rpm - although the surge at 3,000rpm is more than enough to give exhilarating performance. VW says it used the Lotus Elise as a benchmark when developing the EcoRacer, and it shows - this is a proper little sports car.

So will it be built? It's likely, although the carbon fibre might be replaced by a tubular chassis. And there would certainly be a petrol model, probably using the 1.4-litre turbo and supercharged TSI unit from the forthcoming Golf GT.