I'd like to connect a VCR, for playback only, but can't make it work. I understand it should be possible to connect via scart to the Sky+ box, which is itself connected to the TV via scart. The idea being that when you turn on or play the VCR, it overrides the Sky output to display the VCR output. I've connected the VCR to the VCR socket on the sky box, and the TV to the TV socket, but can't make it work. When I hit play on the VCR, the screen goes blank and I have the sound from the videotape but no picture. What's going wrong?