Complex sentence: Churchill found the atmosphere with the Reves' at La Pausa relaxing, with his secretary described him as seeming "twenty years younger" when at the house, which Churchill nicknamed "Pausaland".
Split into simpler sentences: Churchill found the atmosphere with the Reves' at La Pausa extremely relaxing. His secretary described him as seeming "twenty years younger" when at the house, which Churchill nicknamed "Pausaland".