Try that Speedtouch, 510 is ADSL2+ compatible..

The DG534 has been up and down from 9000 to 12000kb/s with attenuation from 16 to 22dB.

Just connected the Billion 5200 and there's an instant improvement.
Downstream Upstream
SNR Margin
6.1 6.3 db
Line Attenuation
17.0 10.3 db
Data Rate
15347 1067 kbps

Thanks for the info on the 510. I'm not 100% sure mine's in good condition. It went intermittent a few months ago so I prodded around inside and changed all the electrolytic caps. This seemed to restore it health but after that it was never quite as solid as the DG534 at 8128kb/s. So I stayed with the DG534 which also has a much nicer HTML UI.

If you re-sync tomorrow well after dawn it will be quite a bit faster.

Why should this happen? I've just re-synced and am getting almost exactly the same speed.

Don't laugh - the sun well above the horizon has a significant effect in reducing background noise. For most installations it varies by 3dB or more.

So if you connect/sync when it is low or or at night you get a lower speed because the noise margin and the background noise both affect the sync speed.

See Noise Margin/SNRM on my website for more detail.

I'm surprised it wasn't better at 9am. Try nearer mid-day, but don't re-sync too often or the BT DLM gets upset. (IIRC and without reading back it is a BT Wholesale connection?)

It does seem a bit unlikely, especially on a line that's entirely underground.. But I suppose anything's possible with cruddy bits of copper with loads of joints down damp holes in the ground. It's still pretty miraculous that ADSL works at all on rotten old POTS lines. It still seems miraculous even though I'm an engineer and understand at least the basics of COFDM, FEC and the other wizardry used to make ADSL happen.

At present, DLM is limiting me to 8000kb/s anyway, presumably because my sync speed has dropped as low as 9000 odd. With a bit of luck my sync will now be stable at 15000kb/s odd so in a few days my DLM profile will get raised.

Dumb me doesn't need to read back on a Plusnet connection - doh!

Profile should rise well within the maximum 5 days with a jump of that magnitude. It would be worth the occasional BT speed test as sometimes that shows a profile rise and the corresponding Plusnet update fails. (Allow several hours extra for that though).

BT speedtest now giving 11343kb/s. Getting better. Perhaps I can expect a little bit more when DLM gets round to it.

Router (Billion 5200) stats seem stable at:
ADSL Firmware Ver FwVer:3.5.10.1_A_TC3085 HwVer:T14.F7_1.0
Modulation ADSL2 PLUS
Annex Mode ANNEX_A

Downstream Upstream
SNR Margin:
6.0 6.0 db
Line Attenuation:
17.0 10.3 db
Data Rate:
14720 1099 kbps

Out of curiosity I tried switching my modem/router to Annex M but it didn't connect at all.