Pretty small, to be honest. Most of it is done using cross-reference of records - if you have a cable or satellite sub, and no licence, then they've got a likely evader. But anyone picking up broadcast TV is pretty undetectable - I really, really wouldn't like to try to design a detector van that could reliably pick up the inadvertent emissions from the circuitry of a modern LCD TV without getting swamped by computers and monitors and so on. In a block of flats - how do you focus on the 5th floor to the exclusion of the 3rd, 4th, 6th and 7th who are licensed?