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Horler's father was a professional trumpeter. David studied piano from age five and trombone from age 14, and attended the Royal Academy of Music from 1963-66. Following his graduation he played with the BBC Radio Orchestra (1967–70), then played freelance in London orchestras, including those conducted by Leonard Bernstein, Quincy Jones, and Michel Legrand. He moved to Cologne in 1980, playing in the WDR Big Band, also composing and arranging for the group. As a sideman, he has played with Maynard Ferguson, Stan Getz, Tony Bennett, Bob Brookmeyer, Mike Gibbs, Clark Terry, Kenny Wheeler, Carmen McRae, and Jim McNeely. He was made an associate of the Royal Academy of Music in 1992.