Trying to find humorous SF/F story, possibly 1960s, in which an automobile traveler (in Britain?) breaks down with a leaking radiator. He is saved by an itinerant tinker, who turns out to be of the Little People. Latter explains that most of the magical creatures have gone for a long sleep in the hills due to air pollution. Tinker doesn't want to help fix the auto until he realizes that more driving will eventually lead to the end of fossil fuels, so his kind can return.