The landscape of West Sussex is timelessly lovely, from the tranquil beauty of the Weald, stretched out between the chalk escarpments ofthe Downs, to the splendid sweep of the coast.   Inland are ancient castles and romantic gardens, unspoilt villages and cosy country pubs, along with Cathedral cities, and a vibrant culturaland sporting scene, taking in opera and music festivals, architectural gems, art galleries, world class polo, horse and motorcar racing.  And sitting magically in the midst of all this is Petworth.  A quaint old market town, hidden among the lush folds of the South Downs.