What speeds were predicted when you signed up?
What is the estimate now, given by http://dslchecker.bt.com/ ?
I wouldn't go by that, BT have a habit on updating that to so that they don't have to do anything when your speed drops.
Our ADSL2+ has had loads of issues reducing in speeds and their dsl checker has been updated to the current speeds, strange thing was Zen used to be about 5.5Mbps to 6Mbps easy with no dropouts, and now its gone to another word for poo.
Sadly out upload sync has now dropped down to about 159 Kbps where as the up stream used to be 1013 Kbps (sync) and BT don't seem to care about upstream and say its all about the downstream, but without the up stream there is no down stream to use LOL