Not sure if this has already been posted elsewhere but the reason there's been more activity on-site and stuff like skips full of waste at the mine entrance is because there were apparently plans to reopen the mine itself as a kind of tourist attraction, doing tours down there and even extending the Steeple Grange light railway from its current terminus (down near the rec/cricket pitch) up to the lorry park over the road.
These plans have been abandoned though due to the discovery of very high levels of radon gas which obviously wouldn't be an issue for the tourists (unless they kept going back every week for years!) but could be a major problem for the staff.
I grew up in Middleton, actually within about 300m of the mine entrance, and am back there most weekends visiting family. I've always wanted to go down the mine and am now kicking myself that I didn't try sooner before security was beefed up. Many years ago me and a few mates wandered into the Hopton entrance having found it by chance, but we didn't even have torches on us so we didn't go very far.
If anyone is up for an attempt then please PM me as I'm game for trying and may know of a way in that people from outside the area are unaware of. I used to climb too and have still got some of my kit so wouldn't be bothered by an aerial assault on it if necessary!