Back in 1955 when it was launched, few vehicles could command the absolute authority of the Rolls-Royce Silver Cloud. Each car weighed two tons and took three months to build. With an interior like a stateroom and a bonnet like a croquet pitch, it was the closest you could get to a car in the style of a country house. But the Silver Cloud was never the sole preserve of the aristocracy. Quentin Willson meets some of its famous owners, including Barbara Cartland and Jimmy Saville.