I've had a look at the ADSL status pages on the router, whilst the connection is stable.
What follows is a bit of a acronym-laden ramble, so feel free to ignore it.
I've got ADSL line speed, attentuation, SNR margin, Errored Seconds, Severely Errored Seconds, Unailable Seconds, Forward Error Correction and Cyclic Redundancy Check.
It looks like the line sync speed has been reduced, as it's now lower than what I've achieved in download tests prior to this knavery.
SNR margin looks reasonable - would be nice to see the actual SNR
Downstream attenuation is twice that of upstream - should they not at least be similar? (~44 dB down and ~26 dB up)
There are numbers for ES, SES UAS, FEC* and CRC on downstream
There are numbers for UAS upstream.
From what I've read, the UAS count should only be incremented as a result of ten consecutive SES occurances. On the basis of which these figures don't make much sense as the UAS count is greater than all the numbers put together. And it's the same value up and downstream.
An engineer visit is booked in for Tuesday.
*Interleaving is currently switched off