during the day because of swearing...after the watershed because of discussions that may prove liable in court (eg. details about housemates friends/family who have not signed their deepest secrets away!)
also if you watch closey {yup I have no life} there are around 7 or 9 mins behind us ,, so its not really life so to speak , therefore they can mute them if they are speaking of family or somthing they would rather keep for the 9 pm channel 4 show.