I got fibre just under 3 months ago, I started with 80Mb. Since then it suddenly dropped to 60Mb, then the next day to 50Mb. My line is about 400m, installed a year ago (new house and road).
Graphs: Sync, SNR. Blank section at the end is downtime on the computer graphing it. 5th November is when I got fibre.
Unfortunately it is fairly common, it's very likely that initially you got good speeds until other VDSL connections were enabled in the same cable on nearby copper pairs. You can see the reduction in SNR in your second link, it's now down to a steady 6dB whereas before it was >20dB even at maximum allowed sync speed. 400m is within the expected limits for 80Mbps but only on such a clean line.
At some point we might get vectoring technology enabled, this essentially knows what is being sent down each pair and carefully adjusts the signal modulation to achieve improved performance. BT has some trials running but no word on a wider roll-out yet.
Edited by rippedcotton (Fri 30-Jan-15 14:15:26)