In November 1977 the London Sinfonietta began its tenth-anniversary season with a performance in London of Transit by Brian Ferneyhough. The concert marks a significant moment in the formation of Ferneyhough’s public persona, just as the tenth-anniversary season marked a turning point for the London Sinfonietta. In this article I undertake a microhistory of this performance in order to ascertain how Ferneyhough’s reputation for complexity was forged in Britain. Rooted in performance and reception history, and based around close readings of key tropes, I show that early critical opinion ranged from sympathetic to ambivalent or curious; yet it never developed into the type of hostility commonly referenced in critiques of New Complexity. The historical detail of this pivotal performance, I argue, suggests the need for a broader cultural history attuned to the transitional state that characterizes both the history and the historiography of new music in the late seventies.