This classy revival of Peter Shaffer’s 1973 play about a boy undergoing analysis after blinding six horses reveals its hybrid nature. Equus is an earnest exploration of belief, a homo-erotic study of repressed sexuality and an occasionally absurd melodrama. Lindsay Posner’s production, which marks Shaffer’s centenary, showcases a performance of glittering vulnerability from emerging talent Noah Valentine as the boy, Alan Strang, while Toby Stephens flirts with hysteria as the shrink, Martin Dysart.