Why am I just getting 0.5Mbit/s download speed?

Question

We have had BT phone and broadband (BT Total Broadband) services for a few years in our household. On broadband, I used to get about 2-4Mbit/s download speed. 1 year ago, this suddenly dropped to about 0.5Mbit/s. For the last year I've been using my internet with a measly max. of 60kbps. I really don't know why this has happened, but it is incredibly frustrating. Can't really use YouTube or iPlayer.
I went on my BT Home Hub settings and found that the downstream is 572 kbps, which corresponds to the download speed I get. Before discovering this, I did a speed test using my test socket but got the same result. A speed test done today using BT diagnostics gives me 0.48Mb download and 0.57Mb upload.
But what's going on? Why is this so? Why did it suddenly drop? What can I do about it? I remember being on 10Mbit/s with Virgin a few years ago, that was more like it. Any assistance would be kindly appreciated.
Here are my stats:
  ADSL line status
Connection information
Line state
Connected
Connection time
3 days, 3:31:19
Downstream
572 Kbps
Upstream
773 Kbps
ADSL settings
VPI/VCI
0/38
Type
PPPoA
Modulation
ITU-T G.992.3
Latency type
Interleaved
Noise margin (Down/Up)
19.2 dB / 12.0 dB
Line attenuation (Down/Up)
42.5 dB / 25.8 dB
Output power (Down/Up)
0.0 dBm / 12.9 dBm
Loss of Framing (Local)
86
Loss of Signal (Local)
7
Loss of Power (Local)
0
FEC Errors (Down/Up)
989552934 / 4294967200
CRC Errors (Down/Up)
730 / N/A
HEC Errors (Down/Up)
N/A / 0
Error Seconds (Local)
1082

Answer 1

Here are my latest stats - connected to test socket, with ethernet cable connection:
ADSL line status
Connection information
Line state
Connected
Connection time
0 days, 0:08:47
Downstream
572 Kbps
Upstream
987 Kbps
ADSL settings
VPI/VCI
0/38
Type
PPPoA
Modulation
ITU-T G.992.5
Latency type
Interleaved
Noise margin (Down/Up)
21.6 dB / 5.4 dB
Line attenuation (Down/Up)
45.5 dB / 25.3 dB
Output power (Down/Up)
0.0 dBm / 12.9 dBm
Loss of Framing (Local)
0
Loss of Signal (Local)
0
Loss of Power (Local)
0
FEC Errors (Down/Up)
22 / 0
CRC Errors (Down/Up)
0 / N/A
HEC Errors (Down/Up)
N/A / 0
Error Seconds (Local)
0