Up to about two weeks ago I had also been unable to connect to
http://www.vauxhallviva.com for about 2 months, maybe longer. No other web sites was affected. I tried using both Internet Explorer (message DNS error occurred. Server cannot be found) and Firefox (message Firefox can't find the server at
http://www.vauxhallviva.com) ,and following the suggested procedures like clearing caches, deleting the “www.vauxhallviva.com” cookie and disabling the firewall without effect. Eventually my friendly neighbourhood guru (thanks Bill) sorted out the problem – by finding a posting on a TalkTalk forum, from HB Peter, who had the same problem in May of this year, which was fixed by changing from a TalkTalk DNS address to an “open” DNS address. Bill made a similar change for me and - problem solved.
My thanks to HBPeter
Later I find that that TalkTalk seem to have only two pairs of DNS addresses so it could be a wider problem.
Mike