Hi ianmonger, yes I experienced exactly the same problem as you describe with BT. I was paying for a package of 40mb download and was only get less than twenty. I found phoning a complete waste of time, I went on-line (Live Chat) where the communication is witnessed (i.e on screen). After considerable exchange of messages BT finally admitted that the problem was externel (i.e theirs') and arranged an engineer visit FREE. Can only suggest that you pursue the same course that I did and "Don't let the *** get you down".
Good luck.