Elizabeth Hurley's son has said his "heart is bursting with pride" following the release of his "big sister's book".Damian Hurley took to Instagram and shared a throwback photo from his time as a child actor on The Royals as he celebrated Alexandra Park's success.The picture shows him sat on the star's lap and smiling as Australian actress Alexandra wraps a protective arm around him. Damian, 19, wrote: "Heart is bursting with pride! my beautiful on screen big sister @AlexandraPark1 has just