While on O2 I needed to have noise introduced to my line due to re-syncs.
This happened a couple of times, so my speed dropped from an initial 17Mbps to 15Mbps, for my line to be stable.
Strangely, while connected to Vivaciti, my SNR is happily sitting at 6db, and my speed is 19Mbps, which is higher than I've ever had while being on ADSL2+
So, a happy bunny here.
Thank you Vivaciti.