Before we bought the house I did a bit of research and got the line broadband tested and was aware that as the house is some distance from the exchange the available speed would be much slower than we had become accustomed. However, having stayed in the pub which is 50 yards away I knew it would be usable.
The service was installed before we finally moved in and had been acceptable for the odd bit of usage it got. However, having moved in the service turned out to be far below what is acceptable - slow speeds, connection dropping when we used the phone and taking ages to re-sync whenver it was reset. After numerous calls to BT they eventually agreed to send someone out.
After some testing he agreed with my diagnosis - it's crap - and set about doing some fault finding. It turned out the the service was fine at the point where it entered the house but not at the master jack. We followed the route and found that the previous occupants had installed an extension by just cutting the cable and taping the wires together in the loft. As well as being illegal it was technically a very stupid thing to do because of what is known as maximum power transfer theory (if you must know its very techie but you cab find out about it here.) Even if he had been able to make very good connections, which you cant by just wrapping wires together, it would have halved the power in the signal, not good when you are a long way from the exchange.
But that is only part of the problems, because we are talking about complicated signals the inductance and capacitance will have been changed (look it up if you must know, but again very techie) and this will have led to a further power loss but also distortions of the line characteristics and the introduction of noise, which is the same as reducing power.
When the engineer removed the extension and crimped the cable properly the service worked well at the master socket, but was still crap in the office. As this was where BT's responsibility ended and the rest was up to me. The nice BT man did leave me with a new master socket, though.
When I followed the extension to the office I found it went under the carpet and then under a very tightly fitted carpet gripper and so had probably been crushed, again not good for for broadband.
Solution: I fitted the new master socket in the loft ( a bit naughty as that is BT's side, but I have the correct tools and did a module of house wiring when I was still in the army) and cabled up the extension properly. Next I installed my WiFi hub and DECT master phone in the loft, to improve coverage.
Result: A very stable and usable broadband connection.
Lesson: If you want a good broadband connection make sure your internal wiring is installed correctly and this is a pretty good guide.