Update on the 'hum'. A Sky engineer came and checked everything out and it's all ok. He didn't hear the hum, because he came during the day, and the hum only happens in the evenings. He didn't think it was anything to do with Sky. I have been making a point of listening for it, and I haven't heard it at all in the daytime. It seems to be during the evenings between about 8pm and 10pm. I am going to have to turn the TV off during those hours for a few days and try and pin it down to specific times.  If it happens at the same time each evening then my money is on the Smart meters. I haven't seen any of my neighbours to ask if they have Smart meters, So it's a bit more investigating to pin the times down. Any developments I will report back!