Its possible that the insataller "back wired" to socket in the dining room so that is was fed from the filtered side of the double socket in the living room.
The way to prove that is to do what I suggested, as that should stop the phone working. You can then replace it again.
Is there any noise on your phone calls? Dial 17070 and select option 2, there should be no noise between the announcements.
I am at work at the moment but I have done the noise test and got no noise on the line.
Didn't know if the master socket needs a filter for the phone.
If the internal wiring has been done correctly, then it should not need one. You can usually tell if its wrong by picking up the phone, the broadband may disconnect, and you will hear noises on the line while it tries to re-connect.
As your broadband is only showing 39 minutes connection time, could that be the problem, or have you been restarting or disconnecting the home hub, as that will be seen as a line fault, and your speed will be reduced.
Its important that you leave the home hub alone, even if you run a speed test.