"The films give a perspective on our great-grandparents' attitudes to sex, which were that it's very much like eating and drinking," says Reilhac. "Sexual desire was a natural thing, and whatever was at hand to satisfy that desire was fair game." The films poke fun at authority figures. In 1925's Abbot Bitt at the Convent two nuns forget their sacred vows over a church altar; a youth spies on them through a hole in the wall and when Abbott Bitt arrives to discover this sacrilegious scene he starts buggering him. In 1920's School Spanking a teacher finds novel ways of disciplining her girls, before the headmaster arrives to prove himself equally ill-equipped to restore order While lesbian scenes are often provided for the titillation of straight men in modern porn films, homosexual ones are rare. But these films were an extension of the clandestine world of the brothel, where society's conventions were overturned. At a time when there was no formal concept of homosexuality as a lifestyle choice, most brothels offered young men as well as women. And the films show strong women in control, leading the way, and apparently taking as much pleasure as the men. Perhaps inevitably, the films were co-opted by the brothels themselves. Knowledge of the blue movies filtered out through the prostitutes borrowed from the huge Parisian whorehouses that the vast majority of French men visited. The brothels were a legal and important part of society that was accepted if not necessarily talked about, and most men had two sexual lives: with their wives and with their prostitutes. As young men were expected to know what to do with their (in theory) virgin brides come the wedding night, elder relatives would take them to the brothel to receive their sexual education. There are letters from Marcel Proust to his grandfather in which the great novelist complains of his father's refusal to fund his weekly visit to the whorehouse. The films were to provide an important function for those yet to be initiated into the sexual life. "The films started to be shown in Paris brothels," explains Reilhac.

"We found printed programmes that showed how they were screened at regular hours in the waiting rooms - young men would find out how sex went by watching them before losing their virginity to a prostitute." Wealthy private collectors, who in turn commissioned more dirty movies, bought some of the films - the grandfather of this respectable family house was one, as was King Alfonso XIII of Spain, who had a passion for women with enormous breasts. In this way the pornographic film industry was born. The reactions to Reilhac's film say a lot about the sexual mores of the countries it has been shown in. French audiences have been made up of couples and friends who have found the film hilarious, but in Spain it has been predominantly single men who have gone to see it in secret. "The word of mouth publicity that we were relying on didn't happen there," sighs Reilhac. "There was a shame element attached to it, which I think has to do with Spain being a Catholic country. The scenes with the nuns and the priest were a big problem because that was perceived as an attack on the church. I had to take out a sequence where the nuns were having penetration with candles. It is illegal to show a desecration of holy objects in Spain."

In the UK the film will be released with an R-18 certificate, the designation hitherto confined to hardcore porn only available in licensed sex shops. This is the first time for over a decade that this sort of film has been given a mainstream cinema release. But the chief concern at early screenings here was over the welfare of a dog that provides a service to a lady of dubious moral fibre. "We had a screening at the Chelsea Arts Club in London," says Reilhac. "Two women were very upset about the dog, and they kept asking if it was harmed in any way. I told them that as far as I could tell that little dog was very happy. Thank God I took out the scene with the duck." Nothing is done to the dog; it is the dog that does the doing. Is something done to the duck? "Oh yes, most definitely."