Many thanks again for your advice. I really didn't think you were being patronising at all
I have temporarily given up and have ditched the router. Now sharing the internet connection between 2 PCs, with no probs except that I can't get MSN Messenger on the client to connect through the firewall on the gateway PC. But what I don't understand is that this router set-up has worked fine for 2-3 years, and suddenly went mad about 3 weeks ago. Something happened then, and we didn't change anything at our end. And 2 new routers behave exactly the same. It's a mystery which I must solve!