Fifty years ago, the dawn of the Seventies, the sound of spring on the British Hit Parade: There was rock (Canned Heat, "Let's Work Together"), soft rock (Simon & Garfunkel, "Bridge Over Troubled Water"), folk-rock (Judy Collins, "Both Sides Now"), pop (Steam, "Na Na Hey Hey Kiss Him Goodbye"), session-group pop (Edison Lighthouse, "Love Grows Where My Rosemary Goes"), Motown (Jackson Five, "I Want You Back"), country (Bobbie Gentry & Glen Campbell, "All I Have to Do Is Dream"), all to one degree or another the soundtrack of the era. But the Number One bestselling record exactly half a century...