I should have made clear that it is a long term memory effect. I am OK at learning song phrase by phrase, better than many. I am not too bad at learning a tune on an instrument phrase by phrase. I play 'from memory' all the time. Its just that the memory is not a set of notes, more a contour, sense of harmonic movement, rhythm etc that has to be matched. But I think it is the thing that Jack Campin suggests SHOUTING.