Thanks for the response.
Odd - he insists that he has been playing on the server withe the two PCs very recently and then it suddenly stopped. We also have people connect from cybercafes that will obviously have the same IP address, and also from the same home. The fact that he will can connect to other servers OK speaks volumes.
I have tried the set net_port ***** command on the desktop shortcuts, but that did not seem to make a difference.
Will not be going the disabling PB route.