Thanks for the reply, Saab Dastard.
The DG834G only has four ethernet switch ports; no dedicated "uplink" port. I can connect any PC to any of these four ports and the ADSL modem to any of the remaining three ports, in any of the possible combinations, and web browsing works fine. Pinging BBC gives a nippy response time (40ms) and resolves 212.58.251.195 as the IP address.
If I instead try and web browse via wireless with the ADSL modem connected to any of the four switch ports, it fails "page not found". If I however then ping the router at 192.168.0.1 (I have left IP allocation at the defaults), it works. If I then try to connect to the router admin page, that too works. If I try to ping BBC however, either using
www.bbc.co.uk or the proven IP address 212.58.251.195, it fails.
The router is correctly allocating IP addresses to each connected PC, so DHCP is working; the first PC gets 192.168.0.2 (ipconfig/all), the second .3, and so on, exactly as the defined Netgear rule states.
No security or parental control options are switched on to restrict outbound traffic.
Cheers.