Typical uselessness

So I moved to Big Bay and had a 20Mbps ADSL line installed around August last year, and it was amazing. Consistent speeds, fantastic latency and the all-round fantastic experience that I've come to expect from ADSL.<br> <br> Come February something on the network changed, what it would be I don't know, but the line has degraded massively. I used to sync at almost 21Mbps but not I'm lucky if I get 16Mbps, which leaves me with a real-world speed of about 12-13Mbps. After months of to-and-fro emails and FB messages they've come to the premises a few times and everything there is perfect, but now they're using the excuse that I'm \too far from the exchange for 20Mbps". Now 2 problems arise - 1) WHY was I getting full 20Mbps until February"