Looks like the Sky DLM is holding them back, so its wait for the system to adjust (a few weeks) or try prodding support now connection is stable.
http://www.coolwebhome.co.uk/calc will give you an idea of what is possible on the line
OK so my trip got cut short, So I've left it a week or two and now have the following stats.
Connection Speed: 4093 Down 1399 Up
Line Attenuation: 33.0db 19.8db
Noise Margin: 15.6db 3.3db
What do you reckon is next? Disconnect all extensions and just use the master for a few days or try and get sky to do a DLM re-learn? I think we still have an issue to be honest with that down stream noise margin. When I plugged it into the master socket nothing changed though.