Bannatyne was born in Dalmuir west of Glasgow. His father Bill had served in the Argyll and Sutherland Highlanders in World War II and worked on the Burma Railway after being captured by the Japanese following the Fall of Singapore, he then worked in the foundry at the Clydebank Singer plant. As a child Duncan lived in one room with his parents and siblings in a large house shared with six other families. He attended Dalmuir Primary School where he displayed a talent for arithmetic and won a place at Clydebank High School after passing the Eleven plus exam. Most pupils owned a bicycle so he resolved to earn the money to buy one for himself. The local newsagent was not interested in employing him so she challenged him to find 100 new customers in return for a paper round. He called her bluff by returning with 100 names, but later reflected that it would have been more entrepreneurial to have sold the list. He only enjoyed PE and woodwork at the High School and left at 15 without any qualifications.