These 15 paintings on paper ought to afford some small consolation to those who missed the Polke retrospective a couple of months ago at Palazzo Grassi, Venice, or a couple of years ago at Tate Modern. Like outsized sketches, their raw, intuitive spirit reveals something of the late German artist’s quit-witted essence. This is painting as a form of liquid brainstorming. Polke pours on the paint, dribbles it into Rorschach spectres, lets it follow in insectile trajectories. There are echoes here of quivering existential anxieties but, characteristically, Polke imbues his pop-infused fear and trembling with a wry sardonic wit. It’s cartoon automatism, here and there infused with a giggle or two. At times Polke mixes powdered mica into his painting’s glutinous iridescence to make it twinkle like a Christmas card. Polke was an incontestably great artist precisely because his talent enduringly bemuses. Plenty of artists have been capable of making this kind of seemingly spontaneous mess, but Polke’s is uniquely charismatic.