Dustin Hoffman’s interest in music – he learnt piano as a child – has come to the fore in recent years. His first outing as a feature film director, Quartet (2012) was set in a retirement home for musicians. In The Choir, which is released on Friday, certificate PG, he’s back in front of the camera, playing a maestro in charge of highly esteemed group of young singers. But London Mums’…