the way to see if its a profile issue kitz is run the BT wholesale speedtest, that reports the BTw IP profile.
Then plusnet have a page in their control panel which reports the line rate (or similiar), that is the plusnet IP profile, these 2 should match or at least be close together.
Agreed.. which is why I first asked the OP to do a BT performance test in order to get his IPprofile... and then when I noticed he was with PN, I gave him the direct link to check his PN 'Stable Rate'.
A slow update is the mostly likely cause & its why I stuck to those questions, but its possible a simple reboot could be all thats needed to pick up the new profile with PN.. but I needed to know if those 2 figures were the same before progressing to the correct answer.
Unfortunately as yet, the OP hasn't returned with the figure I asked for... and Im hoping that he hasnt been deterred over debates over ISPs and PPP