Interesting post on the BT forum, anybody have any further info?
Openreach are aware that some lines do get stuck on sub-optimal profiles, despite line improvements (that court case that BT lost led to changes to the way DLM moves profiles up/down). They are implementing a number of changes to address this, some of which should go live next month.
It does look like ISPs will 'soon' be able to force DLM to reset, but I cannot give you a date. Changes to the DLM algo next month should hopefully make things better for you if you're line is performing well.