You can only merge the STB RF-Out signal with the Terrestrial signal (with the DVB signal).
If you're looking for the SCART signal to be merged with the terrestrial/DVB signal, then I would recommend an RF modulator which can be bought from Maplins for ~£25
http://www.maplin.co.uk/Module.aspx?...=33050&doy=1m9
You can daisy chain the above and it will boost the signal too which is useful for long RF runs (especially if your split setup is in the attic).
But you cannot merge/split the cable that goes from the white box on the wall (where the cable comes in from outside) to the STB as this would go against the T&Cs of NTL's service.
Hope that helps,
Jon