Complex sentence: Of the few runners to contest the big race at Longchamp in the same year, none has improved on the seventh place of Swain in 1997, who had earlier finished a close third in this race.
Split into simpler sentences: Of the few runners to subsequently contest the "Arc" in the same year, the best placing has been seventh. This was achieved in 1997 by Swain, who had earlier finished a close third in this race.