Line/speed in decline - when to raise a ticket

[#1]

So to cut a long story short.. I have fairly regular problems with my phone line, due to a section of the line (beyond my house, council property) running through some trees. The line becomes damaged and my broadband/phone quality deteriorates until service is lost altogether. The section of line gets replaced but the process repeats and after a few years it needs replaced again.
It's been a few years since it was last done and the line is in fairly poor shape. Earlier this year I upgraded to fibre broadband and my speed was a modest 18 Mb, well below the estimate of 30 Mb. It has now dropped to 11 Mb with the odd disconnection, with crackling and noise becoming more frequent on the line.
My question is, at what point can I raise a ticket (my phone is with plusnet too) and see about getting an engineer out? In how bad a shape does a line need to be for BT to consider it in need of fixing? Does the voice service need to have seriously declined before anything is done?

[#2]

Our tests are showing a issue that may need a engineer to visit the property.
If you can go through our troubleshooter below and provide dates for when you'd be available for a engineer we'll be able to investigate further
https://faults.plus.net