Well would not be happy with that out come, Do you talk to your neighbours ? if so and they also have ADSL FTTC it maybe worth while asking them what speeds they are getting, and more importantly if they are
A, Happy with the speeds they get(once you have informed them about the prehistoric BT line plant ) that is causing a serious degradation of service (xdsl)
B, if they would like this fixed, and are willing to sign a formal petition ,should you organise one,
the more people complaining the more chance of BT upgrading the cable , all you can do is make them aware of the facts that they could be potentially being short changed due to this
As for the line not having a fault , it may comply with BT's sin requirements (PTSN) voice, oh and dial up support , But that too is in need of change now that we have FTTC , After all telephone lines where never intended to run ADSL & FFTC services on them, hence why they make no guarantees regarding this
But they don't mind charging ISP'S and SP's for this , If the old line plant is not up to the job,they should replace it, or not sell Xdsl services on it ,