Well, I think the two are not Lord Stanley and Sybil Cadogan, but Duke Amedeo di Savoia-Aosta and Princess Anne de France. See link. :) http://www.savoia-aosta.it/histimage/B009.jpg
That would explain the style of the dress... it took place in 1927.
Prince Amedeo of Italy, Duke of Aupalia and Princess Anne of France (Orleans) - 1927
On 5 November 1927 Prince Amedeo of Italy, Duke of Aupalia and heir to the title Duke of Aosta, married princess Anne of France, daughter of Jean, Duke of Guise (pretender to the French throne) and Princess Isabelle of France. The couple married in Napels: