Quite right Andy, it's a bit of an oiky area, I was up there a couple of weeks ago looking at the locations of the now filled entrances, and a young local youth looks at me, points and says "you do a job don't you". People who work for a living are obviously rarely seen there!
I joined the Dudley Club after most of the filling had happened, but as far as I am aware Castle Hill was filled in as one of the last phases around 1997.
I think the Wrens Nest canal tunnel was dammed to act as a resevoir for pumping in the sand that was used to fill the mines on the Wrens Nest in an earlier part of the works, so is probably totally inaccessible.
A massive amount of local history has been lost through this blanket infilling of the mines, many were dangerous and collapsing, but with a little effort there were areas that could have been preserved.