|Thanks for the reply.|
The set up is as follows:
Cable modem >> (WAN) Router (LAN) >> Homeplug >> various PC's around the network.
None of the PC's have wireless cards fitted (I removed them some time ago).
No particular PC has to be on for any other to connect to the Internet.
All other PC's have a local IP address.
As I said I'm a bit thick when it comes to networks and what I've found this morning is that the problem PC is lacking a network controller ... still trying to establish what driver I need.