Just after I posted I found this: http://www.haskell.org/pipermail/beginners/2011-August/008184.html which gave me the clue I needed.
module Main where
import Happstack.Server (nullConf, simpleHTTP, toResponse, ok)
import System.IO
import System.Log.Logger ( updateGlobalLogger
, rootLoggerName
, setLevel
, Priority(..)
)
main :: IO ()
main = do
updateGlobalLogger rootLoggerName (setLevel INFO)
simpleHTTP nullConf $ ok "Hello, World!"