If you are a Virgin Media customer then the best speedtest is from the link below which is from a dedicated server within the Virgin Media network, hence the is no www. before the address.
Have you tried going to this link to check your speeds
http://gamefiles.virginmedia.com/
click on blueyondergames>demos>then click on at least 2 large files and save them to your desktop, and then wait a couple of minutes then write down the transfer rates on the files that are running> times them by 8 and you will get a truer download speed.