Here's the advice about the Parental Controls PIN. The article includes a comment about watching rated content pre-watershed time (9pm) on Sky Cinema. Sky Cinema is permitted to show rated content at any time but it's an OFCOM requirement to pin enable that rated content if 'transmitted' pre-watershed. Even if you have parental controls switched off.
The workaround is to view Sky Cinema rated content after 9pm then you'll not need to enter the PIN.
I know people say there's no need to pin protect in an all adult household... but it's the OFCOM ruling that governs the 'transmission' of rated content. Sorry about that.