FTTC Excellent but starting to slow

[#1]

Estimated line speed:
    20Mb (Download speed could vary depending on line conditions. Estimates are the maximum speeds that your phone line can support. These speeds are dependent on the package you choose.) - Checked on 2014-07-23 19:47:38
Current line speed:
    17.6 Mb

[#2]

As you say, slightly better, but the results are sort of consistent with the initial ones. Who did your install, was it actually Openreach or one of the sub-contractors? Did they do line checks using a JDSU (hand held tester) before connecting the modem?

[#3]

Yes it was an Openreach engineer that did the install.
Yes he had a hand held box he plugged into the wall socket,  i remember he got the result by text to his mobile.

[#4]

It's beginning to sound as though you just have a poorer quality line, but maybe Plusnet could run some detailed line tests just to be sure there's not a fault on the "copper" (or Aluminium) between you and the Cab.

[#5]

I've done some intrusive testing on the fibre circuit and no faults are found and there appears to be no errors on the line, the copper line test is also passing fine.

[#6]

Many thanks Chris.

[#7]

I don't understand, If my line has no faults or errors why have i lost 1/4 of my speed?
and why am i not getting anywhere near what www.dslchecker.bt.com says i should be getting

[#8]

It's worth remembering that there could have been errors at the higher speed which might have provoked DLM to lower the speed. It's also possible that your initial disconnects may have caused this. Especially if they were in the first 24-48 hours.
Apart from the power off the other night using the method mentioned, has the modem been on continuously, have your previously rebooted it before or since?
What are your technical abilities like?