In the late 90's I went for an interview at a Sony store in London. At the start of the interview the store manager told me that he had been busy preparing the shop's regular report of addresses of everyone who had purchased a new TV for TV licensing. I imagine these were then cross-referenced against who had a TV license for potential follow-up. I struggle to see how the idea of TV detector vans were more than 'enforcement theatre', although possibly a cheap and effective strategy in the past.