To be honest, don't think there is a fault, just a lot of interference.

That is my initial guess.

With the bulging detachable front plate, with a phone jack and one for the router ?

Yes, that is correct.

I managed to get a few snapshots during the night. DSLStats seemed to want to do hourly snapshots which was fine but it stopped before midnight which was a shame. I will have another go tonight if these shots are not enough to get to the root of the issue.

Max Attainable Speed was around 8500 up to 7PM then started to sag just after 8PM and by 10PM was about 7500. I reckon it hit its trough at about 1AM and was still climbing back out on the SNR graph from 4AM to 5AM and was back to normal at 6AM.

6PM : http://www.dcoffey.co.uk/images/misc/Bitloading-2019...
7PM : http://www.dcoffey.co.uk/images/misc/Bitloading-2019...
8PM : http://www.dcoffey.co.uk/images/misc/Bitloading-2019...
9PM : http://www.dcoffey.co.uk/images/misc/Bitloading-2019...
10PM : http://www.dcoffey.co.uk/images/misc/Bitloading-2019...
11PM : http://www.dcoffey.co.uk/images/misc/Bitloading-2019...

Opinions?

Look at them in sequence and you will see teh SNR line in reasonable "smooth" up until the 21:00 image where it starts to become a lot rougher and more so at 22:00 and 23:00. That, to me suggests pick up from radio waves and look, for example, how the notch at tone 602 (2.5-2.6MHz) gets significantly worse.

The real oddity, still, is the total lack of any usable tones from 300 to 520 ...

I realise this is only speculation and may not be how it works in the real world but... up until six months or so ago my connection was more stable and synced at around 8500. Then OR decided to remove a section of the rural poles and bury the cable on about half the run from the cabinet to our cluster, leaving the remaining half above ground on the old poles. From roughly that date onwards, my connection suffered these slowdowns.

I am wondering if the work they did effectively "tuned" the remaining cable to be a better shortwave antenna?

I appreciate there is probably nothing we can do about this issue but do you think it is worth booking a chargeable OR engineer visit to have the tone range and the nighttime degredation looked at?

It could be ... RFI is a "black art" and often not predicatable.

As for a visit, I don't know what it would achieve, apart from seeing what you have. There are a coupe of BT Techs on here who may have a little more idea about what might/could be done.

Don�t bother booking the visit ....

A fair percentage of Openreach staff won�t take anything other than polite interest in your theories.

As long as their PQ test is good, and it tests OK on FT2 (their internal test system) then it�s Goodnight Vienna.

Any thoughts on what might be the cause? Especially that total blank block?

Hmmm ....

A first hunch might be some strange mask or profile, option two, an iffy DSLAM port ?

Course, it could be the OP�s kit not making full use of the tones couldn�t it ?