Slow speeds and frequent dropouts for the last two days

[#1]

Speeds were very bad yesterday. Today, in addition, I have experienced frequent disconnections.
Plusnet speed estimate:
Estimated line speed:
There's no speed estimate currently held on your account.
Current line speed:
0.5 Mb
Information from router (TP-Link TD-W8970):
Current Rate (Kbps): 296 Up 1016 Down
SNR Margin (dB): 6 Up 3 Down
Line Attenuation (dB): 44.6 Up 51.9 Down
BT Wholesale test results:
Download speed (Mbps): 0.46
Upload Speed (Mbps): 0.23
Ping Latency (ms): 223.0
Speedtest.net results:

[#2]

Does your phone still work? Those figures from the router, particular the upstream line attenuation, show something is very broken.

[#3]

Sorry, should have said: yes, phone is working, as is the router.
dick:quote

[#4]

You could dial 17070 and do the quiet line test, to check there's no noise on the phone line.
It could be a faulty microfilter, cable, wiring, the TP-Link router, a line fault, almost anything really. I would try, if possible, connecting to the test socket, using a different microfilter, ADSL cable, and a different modem/router. My TD-W8970v1 did for some unknown reason come with two ADSL cables, but the microfilter that came with it was of a poor design.
Those are the same kind of steps you'll need work through when reporting a fault at http://faults.plus.net/

[#5]

I have swapped out router, filter and ADSL cable, all to no avail. Last night the original router seemed to be perpetually in a cycle of connection/disconnection.
I opened a ticket yesterday, but have yet to hear anything.
For what it's worth, here are the results of the TP-Link's diagnostics tests:
Test Internet surfing
  Test xDSL Synchronization Pass
  ATM OAM Segment Ping Pass
  ATM OAM end to end Ping Pass
  Test PPP Server connection Pass
  Test authentication with ISP Pass
  Test the assigned IP address Pass
  Ping default gateway Fail
  Ping primary Domain Name Server Fail
  Test DNS Root Fail
  Network Connection Status Inspect(NCSI) Fail
Diagnostics completes