I spoke to one of the Derbyshire bat group last night and she told me something similar to what Ferret's posted, that some species can dive vertically and in her opinion, if a human can get up or down a shaft, bats will usually be able to fly up and down it too. The covering of the Clwyd is on dodgy ground legally, and getting NE to look into this is the best course of action - if it can be shown that the developers didn't take proper care to ensure bats weren't resident before covering the shaft, then prosecution is a real possibility.
Follow the horses, Johnny my laddie, follow the horses canny lad-oh!