Dynamic Line Management. Its an automatic tool that monitors your line to keep it in a stable condition. If it sees to many disconnects, it regards it as a line fault and increases your noise margin which in turn decreases your speed.
https://community.plus.net/t5/Library/FTTC-DLM-What-it-is-How-it-works/ba-p/1322799 gives an explanation.
Tracking the issue on BT says it has been fixed this morning but with no details of what (if anything) has been done.
Will run a speedtest when i am home and check to see if I'm getting better speeds.