As someone who works in this field (check my profile). With the 8meg and above ADSL connections there is a 10 day 'testing' phase were the line speed goes up & down while BT finds the speed that is 'stable' for your connection. The speed will be dependent on distance from the exchange and the quality of the cable between you and the exchange. If push comes to shove you can ask BT to switch the cable run for you. It sometimes improves matters (can go the other way too). Also with ADSL their can be cross talk at critical points on the path between the exchange and your house. These can be VERY hard to track down and takes time and persistence by you and your ISP to do so.
Also shop brought phone extensions are nearly always bad news with ADSL as the quality of the cable is nearly always crap, and the cheaper the (extension the worse it gets. There fine for voice use but not for ADSL. If the BT socket is too far from your PC either get the socket moved closer to where you need it, or buy an ADSL router and run Cat5 cable to PC. A side benefit of ASL Routers is you get NAT (Network Address Translation) which acts as a very basic firewall (
http://en.wikipedia.org/wiki/Network...ss_translation) and allows you to have a much more sophisticated network later on should want/need it.
HECTIC PM if you need help getting the ADSL sorted, as it can take time and expertise...!