It being a characteristically wet August bank holiday, we ventured forth to see Plas Cadnant, a house and more especially a garden, which has been beautifully and trimly done up in recent years by Anthony Tavernor, a Staffordshire dairy farmer turned horticulturalist.   It has large herbaceous borders and champion vegetables, clipped trees and below a water garden in a steep dell full of ferns and gunnera.   We wandered round in the rain, nearly the only visitors, admiring the immaculate lawns and planting and longing for the sun.