Don't think that I'll solve anything if i rang BT, do I?
I don't think phoning will help. It's hard enough to find anyone who knows what you're talking about re: the DLM.
The banding can be seen in this line of your stats
Current Rate (kbps) 9997 34999
The 34999 should be 39999. The fact it's such a rounded number (999) and the SNR Margin is way above 6dB tells us the line is capped (banded) at 35Mb. It will sync at exactly that number every time.
The DLM might completely remove the banding taking you to 39999 or it might remove it in stages, taking you to 37999 for example.
Do you know when the sync speed dropped from 39999 to 34999?