This link will give you an extensive lesson on extension wiring. You want to concentrate on the NTE5 master socket.
http://www.readman.dsl.pipex.com/oth...ecatwiring.htm
If the sparky hasn't used the correct tool to secure the wires to the IDC terminals then the wires could easily have broken or just worked loose. Should be an easy fix. Are all the extensions giving poor ADSL performance or just one - that could narrow down the problem?