It sounds like you are on the 300k service not the 768k that you thought. The speed test only mentions 768 kb/s because you told it that you are on that speed. Give NTL a call to check what they have you down as. You could check the config file for your modem.Have a look at
http://www.cableforum.co.uk/board/article.php?a=18 to see how to check your provisioned speed or Robin Walkers site to find out how to check for the config file.