At King Edward's Grammar School, Birmingham, Leonard was only interested in history and English, reading widely.
In the 1930s, Cottrell toured the English countryside on his motorcycle, visiting prehistoric stone circles, burial mounds of the Bronze Age, medieval and Renaissance monuments.
After gaining experience writing articles on historical subjects for motoring magazines, he wrote his first documentary for the British Broadcasting Corporation (BBC) in 1937.
His experiences as a war correspondent formed the basis of his book All Men are Neighbours (1947).
He worked at the BBC until 1960, when he resigned and moved to a house overlooking the estuary of the River Kent in Westmoreland, Cumbria, where he stayed for the rest of his life, writing.