Are super-injunctions illegal?

If one is not even allowed to know that a super-injunction exists to prevent the publication of some material about someone, how can one know that one is about to break it and fall foul of the law? Is it not true that before one can be charged with breaking the law, the law must be at least be "out there" and available to the public, and even if one may not be personally acquainted with it, it should be promulgated for the public to see and take heed. But by its nature super-injunctions are secret and therefore the ordinary citizen cannot know when he is going to fall foul of it. For example, if I had stumbled on the information that a certain person was having and affair with Imogen Thomas but did not know about the super-injunction (how could I?) and mentioned it on my blog, I could be prosecuted for breaking it, even if I did not know it existed. How can that be?