I have a strange extension box in my dinning room, which might be fed directly from the master box (I say might.. the wiring in this household is a mysterious and wonderful thing, and isn't supposed to be understood by mere mortals), but then again it might come off the extension in the front room, or possibly even the alertnative reality under my bed.
Either way there is a 3-way adapter plugged into it, which in turn has a lead coming out of it, and into the dinning room. This lead has a normal telephone plug one end (plugged into the 3-way) and a RJ-11 plug on the other end, which is plugged into a box in the dinning room. This RJ11 socket has a grey lead coming out of it which goes down behind behind a freezer (yes.. theres a freezer in the dinning room... don't ask) and then into the wall, where it promptly disappears

I've spent all bloody night looking for where it comes out, but with no success... the only place left to look is a small cupboard in my room (but my gf is asleep in there and I fear the wrath of waking her) and two 'other things' in the loft converstion which are locked... (rented house that I've recently moved into).
That is, unless it goes back into the old NTL master box

Unplugging the RJ-11 lead brings my noise margin down to 7db (from 20) and doubles my connection speed. (the attenuation remains roughly the same)

Oh well... off to work with no sleep again