Music's narrative qualities have received extended analytical scrutiny in recent years. Less attention has been focused on the characters that inhabit those narratives - the agents or personae within the stories music tells. This thesis synthesises and extends existing literature on musical agency in order to forge a new, analytically productive approach to the critical analysis of instances of agency in twentieth-century British cello music. Through analysis of Edward Elgar's Cello Concerto (1919), Benjamin Britten's Symphony for Cello and Orchestra (1963), John Tavener's The Protecting Veil (1987), Jonathan Harvey's Advaya (1994), Richard Ayres's No, 30 (NONcerto for Orchestra, Cello and High Soprano) (2001, rev. 2003), and Harrison Birtwistle's Tragoedia (1965), this thesis proposes a framework for sustained analysis of musical agency. Theorising the manner in which listeners construct virtual subjectivities in musical works, this thesis proposes a new approach to musical agency in which the subjective and embodied agential responses of listeners are privileged. Focus is directed towards exploring the manner in which musical representations of voice, gesture and volition engage the embodied agency of listeners through mandatory, virtually-mandatory and elective means. It is argued that listeners' subjective and embodied agential responses enable the attribution of virtual agents to musical works. These virtual agents can then partake in agential narratives for suitably inclined listeners. The thesis thereby hypothesises the existence of intra-agency: a conceptual space that blends the various agencies - composer, performer, persona, listener and context - at play in a musical work.