Complex sentence: From 1928 to 1930, he was Principal Private Secretary to the Prime Minister, and from 1930 to 1938 Permanent Under-Secretary at the Foreign Office, where he supervised the work of Britain's diplomatic service.
Split into simpler sentences: From 1928 to 1930, he was Principal Private Secretary to the Prime Minister. From 1930 to 1938 he was Permanent Under-Secretary, where he supervised the work of Britain's diplomatic service.