I vaguely remember this problem from several years ago. I had a site bookmarked and used it regularly and then I suddenly started getting your message (I think - several years ago, memory not what it was). A friend, who is more internet savvy than I am, was able to connect to the site. After running some checks he came to the conclusion that it was my IP that was the problem. I contacted TalkTalk and explained the problem. They gave me a quick fix and, a few days later, came back and said they had fixed it and my original bookmark should work again - it did. It seems the mapping between TalkTalk and the website had got corrupted.
Try getting someone else with a different IP to try the site and/or get in touch with your IP.