1. A method of manufacturing a security document, comprising: providing a first security element article having a corresponding predetermined first unique identification code which is detectable from the first security element article; incorporating the first security element article into a document; generating a machine-readable security feature containing document data representative of the first unique identification code corresponding to the first security element article and of at least a second unique identification code corresponding to a second security element applied to the document; and applying the machine-readable security feature to the document.