Ronald Holden of Cornichon.org tipped me off (that link may be NSFW)—and here's the Guardian's slide show of 16 images from the show. (Oddly, those are, I think, pretty much all SFW.)

I don't know all of what's in the exhibition. The toughest Clarks I've seen are from the Tulsa series (1971) (but here's a tough one from Teenage Lust (1983), definitely NSFW). He's not a flincher, that one.