You need to have the channel selected on 3 (unless your switch is set on 4 on the Atari system). It won't work on Video on the slected TV screen. Now the question is are these systems bought from where you live or did you purchase them from the US? There is a difference in Hz, in the US is 60Hz, and where you are is at 50Hz, which will not get you a picture if you mix the two up.
RE: UK TV set up help needed