Debt collectors
"In the space of 2 months I was down to zero rogue mail, and because of the wonders of electronic banking and billing, I only really get letters about council tax now."
Depends on the last owner. Ours was a mortgagee sale, and the guy clearly has a bunch of debts. So every 3-9 months his debt gets sold on for pennies on the dollar to another agency, who issue half a dozen letters (and charge a few hundred euros), get them returned, rinse and repeat.
So if they owe money, then you'll pretty much never see the end of them. Still getting his mail six years later....