Ongoing Fibre speed problem

[#1]

Product: Fibre Essentials
Line speed estimate: 27.4-36.1Mbps
Current line speed: 18.5Mbps
Signed up to Plusnet a couple of years ago now and line speed was around 28-27Mbps initially, slightly disappointing but acceptable, the line speed would vary between 23 - 28Mbps. About 8 months ago the line speed stared dropping to the low 20's then began to dip down further and further to about 14Mbps.
Contacted Plusnet a couple of times and the second time it was accepted that the line speed was low, after a fault was found and corrected the speed went back up to approx 17.8Mbps where it has remained. This still seems low to me as it is 10Mbps slower than the original connection and way lower than the current  line speed estimate attached.
Nothing has changed since installation, the BT modem is approx 2m's away from the BT Openreach Mk2 Master socket and the router is about 30cm from the modem. I have always used a wired connection and I have checked all wires and re-booted both. Modem and router have always been left on since installation, they are never switched off.
Since the speed problems, whenever I do a BTW speedtest, the download speed bar has change from green to red and the acceptable range is listed in reverse i.e. 20Mbps - 18.5 Mbps, see attached. Also, mentioned at the bottom is the phrase - 'Thist test was not conclusive and further testing is required. This might be useful for your Broadband service Provider to investigate the fault. A TAP3 test produces the same result.
Any help or advice would be greatly appreciated.

[#2]

Your current DLM profile is "0.128M-20M Downstream, Retransmission Low - 0.128M-8.5M Upstream, Error Protection Off" so your speeds won't be getting any higher then that at the moment.
It could be Crosstalk which occurs as more users swap over to FTTC and interference builds up at the cabinet.
At the moment you're just above the low estimate for your line which is 16mbps so we'd struggle to get a fault raised to our suppliers and even then there may not be anything they can do.

[#3]

If crosstalk is an "error" would it help to have error protection ON?
OutaTime  Can you post any stats from your router - which router do you have?