IT looks like any other Volvo V60 estate but hides a silent secret under the boot floor, a mass of batteries that power an electric motor for up to 31 deeply hushed miles.

After that the diesel engine cuts in and this most expensive V60 (by a country mile) becomes a conventional, noisier beast until you've time to hook up the  cable for another four hours of electrical charge.

Actually, there are three way to treat a fully pumped up electric Volvo estate.

Push the button marked Pure and it's electricity only, until the batteries are flattened and diesel power takes over. Choose Hybrid and the car decides which mode is most economical, switching from electric (around town, say) to diesel, for example, when climbing a steep hill.

Thirdly, if you tire of the eco life, a push of the Power button has both diesel engine and electric motor working together, to produce an unlikely looking rival to a full-on sports car away from the lights. The battery charge won't last long like this, though.

So it's clear that Volvo's engineers have thought hard about making their roomy estate as green as possible, while having to use an existing car as the base. In the right circumstances, its high price might even make sense alongside a much cheaper diesel-only V60.

For instance, a commuter with a 15 miles run to the station every weekday morning and the same run home at night, where the Volvo can be plugged in to the custom-installed charging point, or simply hitched to a domestic 13 amp plug socket.

The government is so keen to encourage electric car use it lops £5,000 off the price of the V60 hybrid, making the resultant £43,775 bottom line look a little more attractive, especially if the car is bought for company business and the accountant goes to work on your behalf.

Trouble is, the real world and the fantasy fuel consumption figures that Volvo is forced to print bear no relation to reality. That's not Volvo's doing; its hands are tied by the need to quote the results of tests that come from a standardised procedure imposed on all new cars.

The 155mpg combined figure achieved by the V60 hybrid is both absurdly good and unflattering bad, at the same time. With a fully charged set of batteries the car does an infinite number of miles to the gallon because it uses no diesel at all.

When the electricity is used up, the diesel performs as though you're in a conventional car, actually hampered a bit by having to lug around the weight of the batteries and electric motor.

In precisely 600 miles of mixed motoring, from M11 to central London, my test V60 showed 57mpg on its delightfully clear dashboard readout. That included fully charged batteries from four overnight plug-in sessions, which meant around 120 miles worth of 'free' running.

The space needed for the batteries robs a bit of boot room and the stiff springs needed to cope with their 300kg weight makes the car ride harshly over road imperfections; so the free miles come at a price.

Otherwise, this car is a typically carefully thought out, comfortably seated and well equipped Volvo estate. Packed with safety features too, from active cruise control that keeps your distance from the car in front, to brakes that apply themselves at town speeds if you're slow to react to a pedestrian stepping in front of the car.

Still, a cheaper non-electric, diesel only V60 will makes more sense to more buyers more of the time.