Born in Hoddesdon, Hertfordshire, Pratt lived near to Mike Broadbank from whom he bought his first speedway bike at the age of nineteen, and practiced at the nearby Rye House track.
Leigh was the son of Sir John Leigh, 1st Baronet, of Stoneleigh, Warwickshire, and his first wife Ursula Hoddesdon, daughter of Sir Christopher Hoddesdon, of Leighton Buzzard, Bedfordshire.