A few months ago a BT engineer replaced the master socket in my house and broadband speeds went up to around 12 mbps. Recently speeds have dropped, and seem to be stuck at 6 mbps. I wasn't aware of disconnection problems at 12 mbps so I'm wondering what happened.
Your noise margin, which must have been 6dB or less to get 12Mbps with 34dB of attenutaion has increased to 14dB, presumably targetting 15dB. That strongly implies you had disconnection problems even if you were not aware of them. A few bad hours where you get a whole series of disconnections in a short space of time might have been enough to do it. If you want to know minute-by-minute what is happening to your broadband connection then you need a program called Routerstats (and to leave your computer running constantly). You could ask Plusnet to retrain your line, which sets the noise margin back to 6dB and should bring your speed back up. But you may find it then repeats the previousdownward trend.