“It follows that identifying the measure that the disputed information is "on" may require consideration of the wider context, and is not strictly limited to the precise issue with which the information is concerned, here the communications and data component, or the document containing the information, here the Project Assessment Review. It may be relevant to consider the purpose for which the information was produced, how important the information is to that purpose, how it is to be used, and whether access to it would enable the public to be informed about, or to participate in, decision-making in a better way. None of these matters may be apparent on the face of the information itself.”