Complex sentence: The bank gained clients and grew rapidly in the 1960s and 1970s, its strong work ethic and rigorous intellectual culture standing in stark contrast to the gentlemanly and clubbable milieu of the traditional City houses.
Split into simpler sentences: The bank gained clients and grew rapidly in the 1960s and 1970s. The companies strong work ethic and rigorous intellectual culture stood in stark contrast to the gentlemanly and clubbable environment of the traditional City houses.