Complex sentence: He had a cordial working relationship with First Lord of the Admiralty (and former Prime Minister) Arthur Balfour but largely concerned himself with administrative matters and his prestige suffered when German destroyers appeared in the Channel.
Split into simpler sentences: He had a cordial working relationship with First Lord of the Admiralty (and former Prime Minister) Arthur Balfour. Jackson largely concerned himself with administrative matters and his prestige suffered when German destroyers appeared in the Channel.