It's exactly what the "detector vans" claim to use.

They used to have some of that equipment. Amongst other things, looking for 15625Hz spikes (from the horizontal frequency of PAL CRTs).

I think they phased it out in the mid-80s, or at least stopped replacing the parts when they stopped working, so there very probably isn't any working equipment left now—that's my impression, at least.

Since then it's gradually phased over to essentially SELECT address FROM addresses WHERE residential=1 AND tv_licence=0; now, with a side-order of retailers reporting the addresses of people who've bought TVs (so if you bought a TV, but not a TV licence, you're probably going to get a letter, and probably thereafter a visit).

People who sell television equipment have to, by law, ask you for your name and address and pass it on to TVLA. You don't have to provide them with an accurate address. I'm not sure if ICO has looked at this dual or perhaps misleading use of data holding by shops.

I agree that they probably use their database. I wonder how they get a warrant though? Surely a databse entry isn't enough?