Speedtest.net is erratic at the best of times. Try the BT one, or the thinkbroadband one instead and see what you get.
use this one, http://www.speedtest.bbmax.co.uk/
i find no speed tests are any good for me the only method that does work is using newsgroups, in which i max my line out.
I remember Speedtest.net giving me download speeds of 25000kbs on a 512kbs line! lol.
without being rude thats crap, gives me 300k when i get 2400k with news groups