I don't know the true answer to the problem you have but the solution will very probably be at the TV.
The TV has several inputs -
Regular Coaxial cable from a roof aerial
Scart connector from the Satellite box
Scart connector (perhaps) from the DVD machine
The TV itself will have 2 tuners - one for regular old fashioned Analogue TV and another for Digital signals (DTV)
The Scart inputs will likely be marked AV1 & AV2 on the back of the TV
Patronising now... The TV does not know what you want to watch or which input signal it should put up on the screen. You need to work out which device is AV1 and which is AV2 - Then you need to use a TV feature to tell the TV which input source you wish it to process and display.
There may well be a control actually on the TV iteslf that allows you to do this.
I hope it helps...
Good luck...
Peccavi
October 2009