The only other thing I can think of is that the contention ratio (probably 50:1 if you've got the same Pipex account type that I have) is starting to bite. Simply, it means that Pipex/BT can connect up to 50 users to the same access point in the exchange, as it heads off into the main Pipex trunk.
It may very well be that there are now a good proportion of the fifty slots being used and if they are all on at the same time downloading the latest Kylie album (or whatever) you will see a marked reduction in the performance of your link.
I haven't yet found a way to identify how many others are bolted onto my access point in the local exchange. All I know is that there can be up to 49 others alongside me.
So, next question: does the time of day make a difference to your surfing speed?
ST