It could be a corroded cable pair on the top of the pole outside. Was with me. Openreach engineer went up the pole, chucked me a sample of my cable down for me to look at which he's clipped off and it had the colour and consistency of Stilton cheese. He stripped some shiny copper, double looped it and did the screws up really tight and the problem seems to have gone away. Worth putting the idea in your friendly Openreach engineer's head for sure. Hope this helps someone
