BT Openreach run the FTTC system. If reports in the Fibre Broadband forum are correct, the ISPs have very little control over the parameters of an individual FTTC line.

The good news is that people report the DLM which configures the speed of an FTTC line responds reasonably well to improving line conditions. Once the problem has been fixed, you will likely find the connection speed recovering over the next few days.

However, as more people connect to an FTTC cabinet, the increased crosstalk can lower the speed of an individual connection. This drop is pretty much permanent, though it is hoped BT Openreach will deploy vectoring in due course, which helps to limit the effects of crosstalk.