This might be helpful. It's not truly wireless because it uses your existing mains wiring, and it is a little pricey for just set top box use. Do Sky honestly *need* the boxes plugged in for some reason? I'd always thought it had just been for pay per view/Box Office events, but someone told me the boxes like to "phone home" every so often, a sort of anti-piracy thing. It is in the terms of the Sky contract, but I know plenty of people who leave them unplugged and just ignore the letters. (Not saying you should, just saying it's never hurt anyone I know
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