LONDON — Perhaps the most dominant British athlete in recent history is 42 years old, has a sore knee and spends his days behind a desk in central London helping run a $1.5 billion finance company. 

For nearly a quarter of a century, Tom Dunbar has ruled the game of Eton Fives. The sport, a form of handball which is largely the preserve of Britain’s top private schools, is played on a court modeled after the medieval chapel wall at Eton College. Think squash but played with your hands and with a gothic buttress jutting into the middle of the court.