Have you tried next command?
Also, make sure TCP/IP Properties DNS option is set to "Obtain DNS server address automatically".
No, I hadn't tried that command ('cos I didn't know it!), but I have now and it ran successfully. I also checked the TCP/IP properties and yes, "Obtain DNS server address automatically" is set on. But, flushing the dns hasn't changed anything. Even dropping the connection and re-connecting hasn't altered the DNS addresses displayed in the connection details - the ISP DNS addresses are still showing there. Seems that 'somewhere' they're still being 'remembered'....
Any further thoughts?