The best online listing site (without any doubt for me) is good old Radio Times:
http://www.radiotimes.com/tv/tv-listings
You can use the search box (at the top left) to instantly find any particular programme you're seeking.
Further, you can tailor the listings shown in the table by clicking on 'My Channels' near to the top right. (You can then either select or deselect the channels you want to be shown in the list or, far easier, just click on 'Freeview' [for example] to show just the relevant channels for the TV service you receive).