Speeds are steadily slipping

[#1]

Initially, for the first two weeks of purchasing Plusnet Fibre Unlimited, I had 65mbit.
Then 63mbit.
Then 61mbit.
Now whenever I run Speedtest or BT Speedtest, it's always 58mbit.
Why is my internet going slower and slower as time progresses?

[#2]

If you go to http://speedtest.btwholesale.com/ and run the speed test, click on Further Diagnostics then post your IP Profile results here.