It was a fair old engine, which would suggest it had a fair old engine shaft. It would have probably taken a nadge more than the tip being pushed into it.
My bet would be that it was sollared in solid rock and backfilled. Off the top of my head, there was either no adit, or it was almost at surface. Either way, the rock is so shocking in the area that the adit has probably caved and thus a fair chance the sollars are well preserved in water and will last a while yet.
Having looked up at some exceedingly dodgy shaft caps which inspire one to tie off to a tree when walking around any mine site, I'd say "We couldn't find the shaft" is a dangerous place to leave things!
I imagine that solid rock probably starts a fair way down in the area as the killas is pretty rotten and horrible, from what I recall. I wonder if it could be a choke backfilled to surface.
Either way, it's a lovely conversion and I'd like to see a lot more "saved" in this manner, particularly North Treskerby.