Who is Viscount Severn, the Queen's youngest grandchild?

James, Viscount Severn, the youngest of the Queen's grandchildren, is one of the lower profile members of the royal family. Indeed many may not even have known who he was if they spotted him at Saturday's vigil at Westminster Hall, where he stood alongside his sister and better known cousins, including the Prince of Wales, Prince Harry and Zara Tindall.