>> I'm still getting the fluctuating SNR, although it doesn't go as high as it did when I was on TalkTalk. 7.8db is the highest on BT, on TT it was going into double figures..
Vdsl uses higher frequencies which can be more susceptible to noise. Although looking at your QLN and bit loading, most of the noise appears to be affecting the range also used by adsl. I haven't seen your hlog and can't confirm until I do... but I suspect your issues are more down to noise rather than the condition of the physical line itself.
>>
No idea if my speed will go back to the 39Mb I had previously or will this constant up & down SNR end up with me being banded at my current speed of 35Mb.
As mentioned in my previous post, you are not banded. The line is being limited based on the available SNRM.
As I also mentioned the line rate would be less if DLM applied Interleaving - which it now has and why you have lost some further sync speed.
I hinted in my post that seeing the SNRM graphs would be useful to see if we could determine any patterns. It is noise that is causing the underlying changes in sync rate and also causing Err Secs, which obviously DLM doesnt like.
A zoomed in capture of your bitloading over tones 50 to 500 may also be useful as this appears to be where you are losing a good chunk of your speed.
B*cat is usually very good at spotting noise due to RFI and radio stations... although I suspect from the wider view this is not broadcast RFI as the gap appears too wide.
>> Why also, is somebody expected to pay the same if they're banded at 40/10 & somebody on the same cabinet roughly the same distance can get 55/10.
Because it costs the same to provision the port in the cab and the backhaul fibre where most of the ££££ goes, but if anything it is the longer lines (those further from the cab and the exchange) that are the most expensive for Openreach because they have longer copper lines and require more maintenance of cabling and joints etc.
The longer lines also tend to be more troublesome requiring more Openreach man hours and support for the ISPs.
A line right next to the exchange and cab seldom has any problems and requires short lengths of both fibre and copper, so as far as Openreach are concerned a line capable of 80Mbps is actually cheaper than one which struggles to get 20Mbps.
Since 21CN and FTTC the use of WDM on fibre makes bandwidth a heck of a lot cheaper than it used to be. In the old days of 'just adsl', backhaul was an expensive element because it was in limited supply and why traditionally 2Mb was dearer than 512kbps.
WDM changed all that but it cost Openreach/BTw multi-millions to install the 21CN equipment as the WDM kit is not cheap This is the base backhaul on which FTTC/FTTP runs.... so these days there is negligible difference between a 10Mbps line and a 80Mbps connection when it comes to running costs and provisioning.
Unfortunately it's a fact of life that DSL technology does what it can and goes as fast as it can over copper wires. The longer the copper length, then less the speed