If your eyes have glazed over at the thought of yet another Stanley Spencer blockbuster, unglaze them at once. Yes, all the usual shower of resurrections, regattas and street scenes in (yawn) Cookham are in Tate Britain's show, and, yes, all the stories of easels and prams and lesbian second wives are solemnly trundled out with a faint squeaking sound. But this show, nonetheless, centres itself on something new in Spencer studies, and that thing - brace yourselves - is sex.