A few quick updates here, I've had lots to do away from the computer.
Tweaking the target SNR gives me asmall increase in speed, but anything below 5db SNR significantly increases the error rate and makes things slow down, I'm assuming lots of retries/resends/lost packets. So generally I've left this alone now. (Curiously the SNR is stable, doesn't bob up and down much).
I had a response back from Openreach and from Plusnet, confirming (ambiguously) that "demand is high" in my area, possibly suggesting that it's lack of capacity at the exchange that's now holding me back? Is that possible? There's new equipment installed at my cabinet, but still no capacity due to exchange issues?
I've found plusnet forum very helpful through this, better than TalkTalk and broadly I found their forum support to be good in the past.
No further progress though, but at least it's still working.
If you read my occasional blog, you'll know I'm battling distant-neighbour or stray cats who mess everywhere there's a hint of soil, which has mostly occupied my time recently!
Ian