Great British Gardens: Season by Season with Carol Klein - Carol Klein witnesses some of Britain's most treasured outdoor spaces.     The plantswoman and broadcaster visits Gresgarth Hall, the Lancashire country home of garden designer Arabella Lennox-Boyd. The grounds of the estate comprise the formal garden rooms nearer the house, and a wilder area where Arabella has planted 6,000 trees among cleverly designed meandering paths. Elsewhere, the garden shows off Arabella's immense knowledge and precision. In spring, splashes of…