Where does the problem lie?

[eyemdee]

Okay, I've removed the ring wire from both the master socket and also the socket in the living room (where the micro filter is plugged in) and I'm still getting a noise margin of around 5.6 db:

It should be easy enough to prove whether it is the faceplate or the extension wiring causing the problem.

Temporarily disconnect the extension wiring from the faceplate and test the router in both the test socket and the faceplate. If you still get the wild differences then you need a new faceplate. If not then you need to be looking at the extension wiring.