He was born in Bermondsey, London, in 1912 to Alice and William James Delderfield (c. 1873–1956). His father worked for a meat wholesaler in Smithfield Market, and was the first Liberal to be elected to Bermondsey Council. William supported women's suffrage and the Boer cause in the Boer War. He was a firm supporter of the temperance movement, and of David Lloyd George until the latter allied himself in government with the Conservative Party. From 1918 to 1923, the family lived at 22 Ashburton Avenue, Addiscombe, near Croydon, Surrey. "The Avenue" novels were based on Ronald's life in Addiscombe and Shirley Park.