Complex sentence: At the same time, but back in Brighton, they built a seaside convalescent home for French nationals who were patients at the French Hospital in London; the distinctive turreted structure is now Grade II-listed.
Split into simpler sentences: At the same time, but back in Brighton, they built a seaside convalescent home for French nationals who were patients at the French Hospital in London. The distinctive turreted structure is now Grade II-listed.