I'm <...> getting the absolute minimum from the estimated speed - 3.9Mbps.
I've tried the Plusnet supplied microfilter over my own BT vDSL Interstitial Faceplate and that dropped me to 3.1Mbps.
1. Put the microfilter back and allow the speed to drop to 3.1
2. After a few days, phone Plusnet and complain the speed is now below their estimate. Don't tell them what you did.
3. There is a clause in their contract that allows you to leave their contract early if a) your actual speed is substantially below their estimate and b) you followed their instructions and it didn't lead to an increase. Then you can go back to ADSL.
There is, of course, the other option, where they send an engineer out to your house and he'll find a fault of some sort. Still a win-win situation.