Also, if you use Speedtest.net: http://speedtest.net/
If in the UK, click on the blue pyramid (marked London) next to the recommended yellow one (marked Maidenhead) The London site passed the Virgin test for accuracy but the Maidenhead one failed.
And if you think you are having problems with ADSL speeds go and check out the Kitz site: http://www.kitz.co.uk/index.htm
as it is a specialist ADSL site.