Round here (Pontefract) things the size of a slag heap used to be called a slag heap. Mind you, most of them have been flattened and now are called either an Entertainment/Shopping Experience, or Desirable Housing.
Thanks for the etymology of twmpathau and sleeping policemen. It will save me having to invent an explanation based around a Welsh version of the Wicker Man, where the inhabitants of a remote village had to lure a man of the law and drug him before they could carry out their wild dances in the middle of the road...
Quack!
GtD.