RS
Web has links to more information (some are quite wordy)
Faithnet's page on Natural
Moral Law has an interesting take on Aquinas' theory, leading up to this quote: "In this light it could be argued that Natural Law naturally leads one to Situation Ethics."
Tutor2U's Natural Law page is surprisingly Aristotelian (given that the notes are intended for the OCR syllabus!)
Wikipedia has a clear explanation of The Principle of Double
Effect (but if you don't like their explanation, change it!)