The first moment in the Christmas story is the arrival of the Archangel Gabriel to tell Mary that she has been chosen to give birth to the son of God. Many painters have depicted this event, none better than the great Flemish painter Jan Van Eyck. As befits a man who seems to mixed espionage in with painting for his patron, Eyck’s picture is full of symbols and half-concealed messages. It has an extraordinary after-life - sold by the Soviets against the wishes of the Hermitage and bought by a secretive American millionaire who hid it away in a cellar.