to Cable Forum Marky.
You don't need a cable box to view the 'free' analogue channels on NTL. If you have a service connected, you should be able to get them by plugging your TV straight into the cable (although you will need a converter to make the socket on the end of the cable into the right shape to fit into the TV). We got our free analogue channels this way for a couple of years before eventually subscribing to NTL digital. We sometimes picked up subscription channels too, when they decrypted themselves at random for no reason I could see.
I'd be careful about the Cable box you've been given - I don't know about analogue boxes, but certainly digital boxes remain NTL's property at all times. If it's the same for analogue boxes, you are effectively handling stolen property.