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.
Ah okay. I had already swapped back and all seems okay but I will keep an eye on it.
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.