Quoting a previous Richard Sharpe contribution:-

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Could it be the NTL cable line?? Any suggestions welcome!!

Only a suggestion, Richard, but could it be that the power company has started that thing (can't remember what it's called) of sending data on top of the power lines? As in, is it possible it's now part of the electricity supply?

I'm not sure how you 'prove' this, but what happens if you turn all your power off at the point it enters your house?