David Cameron has done his best to make many things fashionable since becoming PM. Who'd have thought it would be U-Turns that took off as the trend de jour? So of-the-moment, even comedians are adding them to their box of tricks. I'm sure this time last week the Labour-supporting Jimmy Carr didn't envisage himself becoming a poster-boy for the movement, but a few misplaced comments about tax avoidance can do that for a man.