Reading this quickly a couple of things - I'm assuming you are splitting the signal before it hits the STB downstairs, can't really see any other way you could be doing it.
1) The 'interference' is one of two things, either return from the aerial port or the splitter you are using is reducing signal strength to unacceptable levels.
If the answer is that it's return from the aerial port please stop it, this both screws your own service and those of others in your area. To stop this requires a return path filter tho I have no idea where you'd get this from.
If signal strength issue you will require an active powered splitter that amplifies signal.
In any event you are breaking terms and conditions of your contract with ntl, everything is ntl's property, including the coax and signal, until it is leaving your STB to go to your TV.
There, assistance and pontificating in one hit.
