He unscrewed the gas cap and chatted nonchalantly as Colin Friedlos, the proprietor, poured three large jugs of used cooking oil - tinted green to indicate environmental benefit - into the Peugeot's gas tank.

Friedlos operates one of hundreds of small plants in Britain that are processing, and often selling to private motorists, used cooking oil, which can be poured directly into unmodified diesel cars, from Fords to Mercedes.

Last year, when the price of crude oil topped $147 a barrel, a number of large companies in Europe and the United States were spurred to set up plants to collect and refine used cooking oil into biodiesel. The global recession and the steep drop in oil prices have now killed many of those large refining ventures. But smaller, simpler ones like Friedlos's are moving in to fill the void with their direct-to-tank product, having been deluged by offers of free oil from restaurants.

Used cooking oil has attracted growing attention in recent years as a cleaner, less expensive alternative to fossil fuels for vehicles. In many countries, including the US, the oil is collected by firms and refined into a form of diesel.

The oil, Roost said, is "good for the environment and it's cheaper than diesel, even now that prices have dropped." It costs $4.88 per gallon, which is about 10% less than diesel costs now - and about one-third less than diesel cost at its peak last year.

"You can't eat enough French fries" to serve all the cars driven in the west, said Peder Jensen, a specialist at the European Environment Agency. At most, he said, cooking oil might supplant a few per cent of diesel fuel consumption. But he said that it was one of many small adjustments that, added together, could have an important effect on reducing greenhouse gases emissions.