Although they must have some spies as whenever I notify the utilities that a flat has been vacated a letter (from TVL) arrives addressed to the 'new occupier'.
The old occupier probably arrives at his new gaff, receives a threatening letter to the 'new occupier', so changes the address on their licence. This leaves a blank hole at the old address, so threatening letters get sent to the 'new occupier' there.
It's like buying houses, a whole chain of threatening letters gets generated when everybody moves. They must be responsible for half the post sent nowadays.