The focus of London’s musical beam this week shifts to the new Royal Borough of Greenwich where, in the magnificent eighteenth-­century neoclassical buildings now occupied by the Trinity Laban Conservatoire, the annual International Early Music Festival and Exhibition takes place. It’s big business these days. In 50 years, the form has managed to move itself from the fringe to the mainstream. Many people listen to the products of the authenticity movement and accept it as the very definition of “classical”, to use the record-shop categorisation after blues and before easy listening. Once, it was the ­revolution, demanding fresh ears of the Establishment, yet even traditional symphony orchestras now play with “historically informed” sensitivities