I watched a DVD the other night, then watched a bit of TV, and everything was alright.
The next day, I switched the TV on, and nothing happened - just the green number in the corner of the screen (corresponding to the channel) - in this case, it was "AV" as I'm connected to a Freeview box.
I disconnected the Freeview box, and reconnected the aerial directly into the TV itself. By switching through the channels, the sound worked, but with no picture. Just a blank screen.
So basically, I know it's a TV problem (not the Freeview box).