My SNR margin is 29.0 and attenuation 29.5. I've just checked the line capabilities using these parameters plus modem reported speed which is 576kbs on www.snipurl.com/2vfg7 and get the following report:
"Your maximum line connection speed using an ADSL (up to 8Mbps) service is: 4860kbs
If using a BT Wholesale Max product, then your expected IP Profile (sometimes called BRAS Data Rate) will be 4Mbs
The IP Profile is set based on the lowest speed seen in the last 5 days, it drops immediately, but may take 5 days to improve even if you reconnect at a good speed again.
If your service provider uses ADSL2+ the maximum line speed likely is: 16950kbs
With ADSL2+ the maximum IP throughput (result from a speedtest) is likely to be around: 15000kbs
If your ADSL or ADSL2+ connection speeds are slow for the attenuation of your telephone line then take a look at this guide to removing the ring wire from your extensions.
Next step is to remove the bell (ring) wire and then maybe an I-plate - I'll let you know how I get on...

