In message , Dave
writes

Seems that using a detector van can pick out which channel is being
viewed. They can pick it up from the aerial on the roof, though I
cannot see it being so accurate as to be able to discern my aerial
from my neighbour's, three storeys up.

The signal is picked up from spurious emissions from the set and not from
the aerial.
The accuracy of location is very precise at TV frequencies, so determining
which room on which floor is very easy. You could do it with a hand held
receiver at those frequencies.

I think they actually detect the IF from the set's tuner.

No, it must come from the front end of the receivers local oscillator. That
way, they can say which channel you are watching. The set's IF is a constant
for every channel.

I really did mean LO, and only though about it after I had posted

that's alcohol for you

now go switch your computer off and drink some everyone.

--
geoff