Sorry
I very much doubt you will pick up freeview on an internal antena. The freeview system is on low power and will be for a while as it upsets the analog system. To get freeview you will most likely need a high gain rooftop antena.
You current roof antena wont have naything to do with the NTL service, Its most likely just going through the NTL box.
You need to look into the posibility of taping into the roof antena.