Mud Garden

I have an acre of garden ground largely grassed, which over the last few years has failed to dry out as it used to in summer. With the constant rain it is now utterly waterlogged and turns to mud when walked on. The soil is heavy clay down to at least a metre deep and probably well beyond this as it is carse land. Would top dressing with gypsum and sharp grit be of any help and if not what might be the solution?