Astrid Kirchherr Bridget Riley criticism Damien Hirst David Shrigley Dexter Dalwood exhibition Ged Quinn George Shaw Gilbert & George Glenn Brown history Jim Lambie John Stezaker Keith Coventry Leigh Bowery Neal Tait Nigel Cooke Patrick Caulfield Pauline Boty Peter Saville Richard Forster Richard Hamilton Richard Wentworth Rita Donagh Stuart Sutcliffe Sue Webster theory Tim Head Tim Noble visual art