The strange visual world of Paul Nash is coming to seem very familiar. Ah yes: a bright red fungus levitating over the sea (or is it sinking?); giant metal girders in a summer cornfield; an ethereal snake in the woodshed (and an extra snake in the sky). Tate Liverpool staged a big Nash show in 2003; Dulwich Picture Gallery mounted ‘Paul Nash: The Elements’ in 2010. If there’s a canon of British painting, Nash is in it. The surreal ingredients of his art have become part of the national furniture: clouds as solid as flints, ladders leading nowhere, stone circles thinking about coming to life.