Our nearest transmitter is up on the Preseli Hills, some 35 miles away. I suppose that it still could have an impact but confess to being dubious. Both Digital TV and analogue radio signals are very poor here. Both 'drop out' frequently at least partially because we are down below a local hill line and well out of 'line of sight'. Additionally the bulk of our cable run is underground.
My local set up is through the BT NTE5 socket with the No. 3 ring wire disconnected. I have cut off all of our 'phones a couple of times and tried overnight straight into the test socket but the results were more or less identical: can't do that too often as I get complaints from, well, I'll let you guess who. The point is though, that I can more or less eliminate all domestic wiring as a source.
As I said, we are quite rural and I understand from a friendly BT engineer there are not actually many connections on our particular bit of the exchange internals. SNR seems to be such a grey area with so many variable I can't see how anyone could properly understand what is actually going on. Professional or, like me, amateur.
Thank you for taking the time to reply though.
Cheers
Alan