The best the NTL boxes are capable of is Dolby Prologic. This is 4 channel surround sound through a stereo connection. Dolby digital (for 5.1 surround sound) requires a digital connection, which is not present on our boxes.
Any film which is stereo, probably has a prologic surround soundtrack though, although the amp will need to be switched manually to Prologic mode as most Amps cannot automatically pick it up.
NTL should update the boxes though..