Simon,
I do not doubt there is interference that is causing the loss of speed, however i have done as you have asked me and plugged my router into the test socket thus eliminating any noise that could be being introduced by anything i've done within my house. Therefore the only other source of noise should be between the exchange and my property. As my neighbour has not had a decrease in speed then it is reasonable to further assume that no noise is being introduced on the bulk of the line length. Maybe it is a loose wire or damp, or gremlins at my property junction box, i don't know, but what i do know is that if i tamper with anything outside the property then i am acting illegally.
Andy
Andy,
Whilst you may have reduced the incidence of noise, plugging into the master socket does not eliminate it. You still have a wire between the master socket and the router, and the router itself, which can all act like antenna to pick up noise. The trick is to identify the noise and then eliminate it. I spent 3 months a year and a half ago trying to identify the cause of random noise bursts on my line which eventually crippled my connection to 250k. I tracked the source of the noise to my monitor on my 2nd PC. I swapped the monitor out, contacted IDNet and reported my findings, and they reset my line. Problem went away.
There are tricks you can use, such as walking around with a detuned radio to listen for the noise, changing cables, buy the more expensive shielded cables, try a different router etc. I recommend trying all of these to identify the source of your problem, and eliminate it. Only then will you be able to recover your previous performance, or to be able to eliminate internal problems. Once you are confident that you have no internal issues, then BT have a responsibility to fix it.
Do you have a streetlight outside your home? These have been known to cause noise problems, too. Hell, I have a freeview box (now unused) which was sensitive to cars driving past. Noise can occur from a multitude of places and only time and perseverance will help you identify where.
Steve