The problem is that the modulator output frequency of your sky box is exactly the same as for example chanel 5 on your terestrial aerial.
You need to change this number to a different one thats clear of local terestrial transmissions
1) Select the Services button on your Sky or Sky+ remote control.
2) Type in 401 [select] on your key pad and you will get to the installer menu. You cant gain access to this via the drop down menu as it is designed for the Sky Engineer to use.
Select Option 4 to configure the RF outlets on your Sky Digibox.
4) The option RF Chanel Number change this number from example 35 to 60 .
You may need to do this using other numbers 26 34 55 etc
Every time you alter the number store it then try & tune TV to sky.
Any other sets in the house connected by RF leads will require retuning for sky.
Save new settings and you should find that your RF outlet to TV works fine.