Whatever I did, I could not find the problem. The nearest I got was that it stopped when I turned the power sockets breaker off! In the end, I gave up, just hoping it would go away. Eventually, the continuous torrential rain stopped, and so did the intereference! Maybe it was something to do with the high voltage pilons about half a mile away getting wet. Don't really know, but it will be interesting to see if it comes back when the rain starts again.