Meanwhile, someone else has advised me as follows:-
TV detection with the 625 Line system was based on the line timebase high voltage transformer LOPT's (Line Out Put Transformer) electromagnetic field. It produced considerable radiation; for a good 50 feet or more. So using a BBC receiver within the van to monitor the off-air signal via its line-time-base on a 'scope, you could detect the radiation from the TV within a house, and identify the channel it was receiving by comparing the two signals. Today however, it's impossible to detect any useful signal from a flat screen TV, where radiation from any local oscillator is well screened, and no line timebase to detect any more. What TV's, and other domestic products, radiate is wide spectrum hash-noise. And if these also use SMPS's (Switched Mode Power Supply) - they wipe out most HF Short-wave reception and low band too.
For more ... see this