Complex sentence: Vaughan Williams worked on the "Lark Ascending" prior to the outbreak of the Great War, but there is no reliable evidence to support the claim that he was working on it while watching British troops embarking for France.
Split into simpler sentences: Vaughan Williams worked on the "Lark Ascending" prior to the outbreak of the Great War. There is no reliable evidence to support the claim that he was working on it while watching British troops embarking for France.