The late prom brought the Estonian Philharmonic Chamber Choir and their conductor Paul Hillier for a concert celebrating the 70th birthday of Estonia’s most celebrated composer, Arvo Pärt. His works were interspersed with others far older, including a brave and effective run by the men of the choir at Pérotin’s 12th-century Sederunt Principes. With the organist Christopher Bowers-Broadbent, the choir did full justice to Pärt’s own pieces, of which his motet-like narration Dopo la Vittoria came over best. But for a composer who attempts to strip away the unnecessary, it was striking that his works should seem no more streamlined than those written hundreds of years before.