My first suggestion would be to plug your modem directly into the BT Test Socket (found by taking the faceplate off your master socket), turn it on, leave it for two hours and then see what speeds you get.
Here are some links that have proven invaluable to me in learning about ADSL and how to improve my connection speed. Most of the time the problem lies within your house, and a few simple tweaks can vastly improve your speed.
http://www.beforum.co.uk/showthread.php?t=8152
http://www.kitz.co.uk/
http://www.thinkbroadband.com/faq.html
http://community.plus.net/blog/2007/...w-to-diagnose/