You haven't missed a sitter - see the last definition in Chambers under the entry for 'setter'. The word is old (17C-19C) underworld slang for a spy or a police informer.
That's a relief. Thanks, jacknatter.
The silly thing is I checked setter in Chambers p1428, and then checked under set on the previous page as directed and still didn't spot it! Have now though.
Isn't set renowned as the word with the greatest number of dictionary definitions? That probably applies to the OED.