If you're really intent on getting the best connection, then you need to replace the master socket with a filtered faceplate (preferably the ADSL Nation one), and connect your router directly to the ADSL output of that. Connect the extension to the filtered output. Then run some ethernet cable from the router to your PC. This will completely eliminate the effects of the extension wiring, and you won't have to worry about fitting microfilters to your phones.
Home ADSL: Be Unlimited LLU
Be LLU supplied by Andrews & Arnold
Orange 3G Business Everywhere Mobile BB