Though connected on (and receiving) a 2MBps service, my router was showing downstream SNR of as low as 2DB. We were getting decent enough speeds, though, so obviously someone at PlusNet thought that it wasn't a problem. I thought it was odd, but let it lie.
Since the regrade from 20:1 to 50:1 contention, the SNR has jumped up to 14-16DB! And the connection speed and quality have (obviously) improved. Is this supposed to happen? Or have BT done some maintenance somewhere that has caused this?