Basically, you want a Wireless "Cable/DSL" Broadband Router. This will connect by ethernet cable to your existing NTL Cable Modem (or Set-top box) and your PC1 can then connect to the router with another ethernet cable. Install a "Wireless Network Adapter"in the second PC and access the router wirelessly with that. Each PC will have internet access independently from the other and only one PC needs to be running.
A word of CAUTION:
Be very careful NOT to get a combined "Router/ADSL Modem". These are not suitable for NTL's Cable broadband, you need a "stand-alone" router often described as a "Cable/DSL" router (don't confuse the reference to DSL with the combined router/modems).
<<< If I boot up both PC's can I share at will? >>>
Yes, with a router you can share internet access, Files and Printers.
<<< do I have to get the NTL people in to give me another 'entrance' into the cable >>>
No, the router will just connect to where you currently connect the PC.