[jaycee]

I have read it is mostly to do with when they were required by some local councils to provide an unencoded analogue feed to certain areas. If you read somewhere else on these forums, an NTL engineer goes into how this is one of the biggest problems with noise on the network (since as inside TV's pick up all kind of rubbish) being injected into the system from direct, unisolated connections to TV RF sockets.