During the 1950s and 1960s in London, in the Royal Festival Hall, an unusual series of concerts took place. These concerts stood apart from the usual offerings in London’s post-war musical life. What they offered was early music, principally J.S. Bach's concertos for two, three and four keyboards, played not on the piano, as had hitherto been the case, but on the harpsichord. This article documents, for the first time, the facts, and the implications, of the Royal Festival Hall concert series: how it came about; the repertoire; the performers; and the performances. The article concludes that the Royal Festival Hall concerts were notable in the evolution of the early music movement in the UK, deepening its reach to a broader audience and nurturing an awareness of an issue that was increasingly to gain traction in the later decades of the twentieth century: the idea of historical authenticity in the performance of early music.