David Shrigley is a mordant and rueful artist. I have a postcard by him that says DEATH in letters that get smaller from left to right, pithily expressing our doom as a diminishing scribble. His take on existence veers between the grimly comic and the cynically absurdist. It is therefore hard to take him entirely at face value when he claims his colossal bronze sculpture of a hand with an elongated thumb jabbing the sky above Trafalgar Square is a simple statement of optimism. I honestly can’t see this gleefully ugly work of art spreading a lot of cheer.