[Chris]

So, when you say 'mainstream' channels 1-5, presumably you mean analogue signals from your aerial, then when you go onto channel 6 you're getting input from the box?

Sounds like either your channel 6 is badly tuned on your telly, or one of the cables in one of the many boxes you have is not properly seated.

Also, if you're using channel 6 for your set-top box, I'm guessing that means you're not using something nice like a SCART to join your STB to your TV? You would get a much better picture on all your cable channels if you invest in a SCART cable. You would then use your TV's AV channel to view cable TV.