There has been some griping about the preamble, in particular regarding the word "zigzag". Can't see the problem; the entries do zigzag, ie "move forward making an alternation of short, sharp turns" (Chambers). As it is made clear that none are entered moving upwards, it doesn't take long to understand how answers are to be entered.
However, the penultimate sentence of the preamble is barely English, and I suspect written by an editor, not the setter. Anyway, it's the best Listener for ages. The clueing was eminently fair (despite traps for the unwary, of whom I am one) and the endgame was brilliantly executed. It was a pleasure to have a proper challenge after some very weak puzzles recently. Strange, though, that the same writer should reappear again so soon. I had hardly heard of him before he turned up in April.