Update for anyone interested (just realised this is being posted a year after my last post): I think the mill and land have been sold. There is work being done on the buildings. I went there last weekend with my son to check out the culvert. All ground floor entrances have been bricked up properly. However there is still a small hole in the wall on the side where the chimney is that you could get through (I'm 6'2" and about 16 'n' half stone and if I took my big coat off I'd probably get through it
).
Looks like the engineers are looking for the sewer pipes and lots of drains and 'manholes' were uncovered all around the main building.
Also:
So this is the point of interest that I've not seen anyone investigate (i.e. go wading through). At this time it' is, relatively speaking, more a river than a brook and you'd probably get taken off your feet trying to go in. I suspect you'd also need a gas meter. The second to last picture shows Ovenden Brook running towards/under the Mill; last picture the culvert running back up the valley towards Halifax Boxing club - where you can see it come out into the open as a brook, then go under the club.
Apologies about the quality of the pictres, I was using a mobile phone as I don't own a proper camera.
Lastly, the picture of the strip light you can see some sticks taped together to make what looks like a stick figure. Maybe the local kids, high on sugar and caffeine, got creative