The Orphan Choir - Sophie Hannah

Louise's seven year old son has been sent away to boarding school against her wishes, and she misses him desperately.
And her neighbour from hell is keeping her awake at night by playing loud, intrusive music.
So when the chance comes to move to the country, she jumps at it as a way of saving her sanity. Only it doesn't.
Because the music has followed her. Except this time, it's choral music sung by a choir of children only she can see and hear ...