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Um...Although I'd agree with some of the views here, they're not really answering the question.
Thanks for pug's links. Especially intrigued by one quote from the BBC:
"And once the television is detected, the equipment - which works from up to 60 metres away - can pinpoint the actual room that the television set is in."
Now this might be the crux of the matter: I used to live in some flats, half of which ran east to west, the others north to south. So even if you could locate a signal in three-dimensional space, that in itself wouldn't tell you which property the signal came from. Unless the detector vans have the architectural plans of each block of flats in the UK, I can't see how that they're much use. At least in blocks of flats.