"There are few hours in life more agreeable than the hour dedicated to the ceremony known as afternoon tea." Henry James Portrait of a Lady.   Given the popularity of National Trust tea-rooms, visitors would seem to echo James' sentiment. Jane Pettigrew has collected recipes for cakes, breads and biscuits from National Trust cooks and chefs to provide a survey of regional dishes that use local ingredients: for example, Old Peculier Fruit Cake from Yorkshire, Bara Brith, the traditional Welsh tea bread, and Irish potato cakes. Tempting goodies also include sticky lemon cake and chocolate orange drizzle cake.   Examples of tea's associations with National Trust properties are legion and in this book Pettigrew discovers curious and ornate tea wares from the Orient and from Europe, relates stories of bizarre tea parties, and finds that each afternoon at Lanhydrock in Cornwall a tea table is set with lace cloth, fine porcelain, silver tea knives and linen napkins - just as tea tables have been set in Britain since the late 19th century.