'Plunk'.
John Mortimer was the artist on several features for Buster in the 1960s. He illustrated among others 'Joe's Transport Cafe' (1964-65), 'Ocky' (1965-66) and 'Plunk' (script by Roy Davies, 1965-68).
He should not be confused with British short story writer and TV screenwriter John Mortimer (1923-2009) or another TV screenwriter, John Edward Mortimer (1930-1992).