This paper describes the Interactive Document Summariser (IDS), a dynamic document summarisation system, which can help users of digital libraries to access on-line documents more effectively. IDS provides dynamic control over summary characteristics, such as length and topic focus, so that changes made by the user are instantly reflected in an on-screen summary. A range of 'summary-in-context' views support seamless transitions between summaries and their source documents. IDS creates summaries by extracting keyphrases from a document with the Kea system, scoring sentences according to the keyphrases that they contain, and then extracting the highest scoring sentences. We report an evaluation of IDS summaries, in which human assessors identified suitable summary sentences in source documents, against which IDS summaries were judged. We found that IDS summaries were better than baseline summaries, and identify the characteristics of Kea keyphrases that lead to the best summaries.