In reply to TobyA:
Just back from a dusk stroll down the bridleway through the quarry - there's some wreckage still piled up against the wall. But since the view seems to be that it was a lightweight structure on a lorry chassis, perhaps it won't have made too much of an impact. I can believe, though, that they built up the quarry floor a bit there to create the pond the thing landed in. The planning permission said they had to restore the quarry to its original state, so presumably the pond will have to go, though how much they'll actually restore the bottom of any routes remains to be seen.