Hewer was born in Swindon on 17 February 1944. His mother was Mary Patricia Hewer ("née" Jamison; 1918 - 1999) and his father, John David Radbourn Hewer (1915 - 2010), was a senior partner of Hewer, Spriggs and Wilson, a veterinary practice in the Old Town area of Swindon. They met when both were university students in Dublin. The family lived in Old Town and Hewer was educated at Clongowes Wood College, an exclusive Jesuit boarding school in County Kildare, Ireland. Hewer has two sisters and two brothers.