It sounds like you're using a proxy server (OpenDNS) and it's actually that that's blocking the website. As far as I'm aware, OpenDNS do not have any servers in the UK, so it seems like it would be an odd idea for TalkTalk to use this as a default setting.
What operating system are you using? If you're using XP, then go to Control Panel > Network Connections > right-click the connection that relates to your internet access. Then click the "Properties" button. Under "This connection uses the following items" highlight "Internet Protocol (TCP/IP)" and click "Properties".
At the bottom of the window it will show two radio buttons, one says "Obtain DNS server automatically". The other forces your computer to use a pre-determined DNS server, such as OpenDNS. Make sure the option is clicked for obtaining the DNS server automatically and then click ok. Report back if this doesn't solve your issue.