9pm was the original baseline for the "watershed" i.e. time before which it was considered safe programming for kids to watch. since then there have been other guidelines, but not "set in stone". i.e. it is advised not to show anything which may shock or make a person feel ill, during "tea time" (like the insides of a pet, for example on "vets hospital" or whatever it's called - that god-awful programme with Mr Wobbleboard Harris) unless it is on a programme that the viewer would expect to see that sort of thing (i.e. watching violent scenes in a news bulletin, or blood in a hospital drama). I think they're allowed to relax things a bit after 8pm i.e. show mild violence (ala the Bill) and casual swearing. Anything pornographic in nature has to be shown after 10pm. There are other guidelines but i don't know them all, you'd have to get them from the Beeb or the ITC