It appears Plusnet have had a few issues with DNS servers.
This has caused my iMac (MAC OSX Tiger + Safari) to not recognise any web addresses by name only by IP address.
I've entered the IP addresses of the DNS servers in my router (DLINK 604) as well as in the macs network preferences but all to no avail.
I can connect via ethernet on my PC so assume the connection is OK and the problem is limited to the set up on my mac (i've not changed anything prior to the problem so I guess it's linked to the DNS problems Plusnet say they've had)
I can connect via my neighbours wireless so I know Safari on the mac is working