I use Samknows to monitor my Broadband connection and have enjoyed 28Mbps download (which is what I would expect given my distance from the street cabinet) until recently.
I have attached a screenshot of my daily download speeds since 1 October to illustrate this and would be pleased if someone could advise what is happening.
In addition I have a BT Smarthub and have noticed that my noise margin has changed from 5.8/3.0 to 5.8/6.0.
This matter has been reported to BT who say that the line is performing within their parameters, and in any event I have a guaranted 20Mbps.