It's the risk you take, but if you can prove the speed drop you've got a leg to stand on. Another good one is to dig out the original mail you got from BT promising you a certain speed, then compare it to what you're getting now. I did that, and they offered me an instant out of my contact with no fees as due to the current state of the cable they can't deliver what they promised. Nor is it cost effective for them to dig it up to replace.
I'm with adsl24 and they are smarter, not promising anything.
My speed is far below the wholesale checker if that proves anything?