Censorship on films question..

I know i've asked this question before, but i've been confused yet again.

I was watching Back to the Future 2 today, it finished at around 6pm, yet the film had words like "Son of a B*tch" and "Sh*t"

Yet I remember watching Liar Liar not so long ago just after Coronation street and that having all the expletives, even down to b*tch removed.

What's going on here? I thought it was only Sky who got away with saying such expletives in the afternoon showings.

Well BTTF2 is rated PG while Liar Liar is a 12 so the former has been decreed safe to broadcast at pretty much any hour (even more so on the Sky platform by virtue of less viewers), the latter went out mostly before the watershed which of course is a no-no when it comes to profanity.