This morning, it was working fine. I encoded a video quickly, then once it was finished, I shut the lid (i.e. put to sleep) for a bit, then when I came back to use it, the screen wouldn't turn on. I tried restarting it, resetting PRAM, SMC and the screen just remains blank. It does boot up (chime at the start, hear the hard drive doing stuff) just the screen doesn't come on. So I hooked it up to my external display and tried, no luck. The fact that the external display doesn't work makes me think it is definitely the GPU and not the screen (I had weird issues with my screen sometimes, had to push the screen, as if the cable was loose). I'm going to take it into John Lewis tomorrow for repair (first year of Apple Care gone, on my second year via 3rd party), but was going to see if there was anything else I could try before flying in with the assumption that my GPU has failed, after having my MacBook Pro since July 2007.