I'll have to see if I can quantify the effect I think I am seeing.

I'll start logging error counts against time and compare with my usage patterns to see if it's all in my mind or not.

I just had a browse in the TalkTalk forum, and it seems that some posters have been asking for their noise margin to be dropped from 15dB to 9dB with very successful results. So it looks like at least some people in their customer support team must understand how to get this done for you. Why not give it a try, and post back here with the results.

Just came back to this and found this chart, showing a typical effect of Annex L in yellow. Whilst you always need to take these general charts with a large pinch of salt for any specific case, 52.5dB attenuation puts the OP around the 4 - 4.5km mark which is comfortably within the range benefited by Annex L. The actual stats suggest that this is quite a noisy line as well, so this would tend to push the OP further into the beneficial range.

useful chart for relative expectations, thanks.